Again, as a newcomer here, I'm not sure if I'm posting this in the right forum, and again, I ask that if I've erred, could a moderator please move it to the appropriate place. 
Essentially, I'm working on a planar human character (most likely from Sigil) for a 3.5 Planescape Campaign using the Factotum class from Dungeonscape, and later hoping to go into the Chameleon Prestige Class from Races of Destiny - the build will likely end up being Factotum 8/Chameleon 9 then back to Factotum for the last three levels. For those who don't have these sourcebooks, Factotum is a jack-of-all-trades, trapfinder, skill monkey (having all skills as class skills), "5th character" type class which uses a new mechanic called "Inspiration Points" to gain bonuses to their dice rolls, cast spells as spell-like abilities and emulate the roles of some other classes. Chameleon takes this to a whole other level, being entirely based around the concept of mimicing other classes abilities, including granting arcane or divine spellcasting abilities (and at higher levels, potentially both simultaneously).
Anyway, mechanics of the character aside, I've come here for advice and input on fleshing out the story/background side of the character, since I'll freely admit my knowledge of the Planescape is limited (though growing).
Basically, the character concept is a planewalking rakish/scoundrel (chaotic neutral alignment) dedicated explorer/adventurer/treasure hunter jack-of-all-trades, who makes his living taking contracts from an adventuring/explorers guild or freelancing (with adventuring parties, as a mercenary, whatever). His primary motivations are coin and cleavage, with power and reputation being means to achieve these goals 
. He's also a writer (reflected by ranks in Profession: Scribe) and a story-teller (reflected by ranks in Perform: Oratory), and uses these abilities to spread exaggerated and enhanced legends/rumours/tales of his adventures/abilities and expand his reputation.
Here's what I've got so far in terms of background story:
- Born the bastard son of a noble household, the result of an illicit affair between the male head of the house and a female noble of another house. Out of guilt, the nobleman took the boy as his ward under some pretense, though the nobleman's wife knows the truth and resents him and the boy for it, and many other nobles have guessed/learned the truth. His father sees him as an embarassment and the cause of much conflict in his marriage.
- The nobleman and his wife have no children of their own (his wife is infertile). Though in effect the first-born son, because of his unique origin he is ineligible as heir. Still, he grew up with a fairly privileged existence, with access to excellent tutors in a variety of fields and was given an excellent education.
- As he grew older and became frustrated by his position and the bleak outlook for his future (resented by both parents and unable to be given the wealth, power or respect befitting the heir of a noble household) and lured by tales of planewalkers, he left in order to join an adventuring guild/organization of some kind. Adventuring offered a means of attaining the status/power/wealth he was othewise ineligible for.
Does this sound feasible? I'm not sure if this kind of "scandalous" background story works in the Planescape setting - in a setting where fiends walk the streets with celestials, is the story of an illegitimate heir even considered a scandal? Any suggestions, additions, changes or comments?
Some more questions on his early life: Where might he have been educated? Is there some sort of well-reputed college or educational institution in Sigil or otherwise on the planes he might have studied?
For his transition to adventuring: Is there a guild or organization of dedicated explorers and adventurers on the planes? Or perhaps an organization dedicated to chronicling tales? A lot of the stuff for Factotums discuss adventuring/exploring guilds/lodges - is there anything like this in Planescape, or will I have to make something up?
Any other suggestions, comments or additions? Recommended reading? Anything at all? Feel free to ask questions and I'll do my best to answer.
Any and all responses are much appreciated - thanks for your time.
Factotum 8/Chameleon 9 then back to Factotum for the last three levels. ... "Inspiration Points"...
Welcome!
A Factotum is a particular term in Planescape, meaning someone who is active in a faction, one of the philosophical groups that are the primary bodies of influence in Sigil. They're low-ranking but more official than a "namer," who simply declares an allegiance to one of the factions for political convenience' sake. Factotums really adhere to and seek to advance the faction's beliefs.
"Inspiration Points" are a version of what Planescape called belief points, though in the original form they were rare and powerful, without much in the way of set mechanics. Still, your character description fits very well with someone who is vigorously active in one of the factions, believing it wholeheartedly to the point that his belief provides him with benefits in his life. Which faction would shape your character significantly. I suggest downloading our 3.5e version of the Planescape setting, and reading through the list of factions to get an idea of which appeals to you.
Nope! :^) Sigil has no hereditary nobility. The factions are the primary influence groups. And there are so many byblows and halfbreeds running around the streets that people categorize them by type; it's a town where lots of different sorts come together. Your best bet for preserving your family story is if your father is rich due to connections to nobility on some Prime world; while his rank holds no meaning in Sigil, his money certainly does. Now, many human cultures have laws for inheritance when a marriage is barren, varying from divorce and remarriage to legitimate adoption, so your disinheritance would depend on your father's home culture. If you are the result of a union with a fiendish temptress such as a succubus or yochlol in disguise or dominating your father, your homeworld's laws might have rejected you even if you lack any apparent taint (in the form of a tiefling or other template). This would be irrelevant in Sigil, but could certainly still sting.
Very little in the way of formal education exists in Sigil. Private tutors hired for the purpose would be most likely. Someone with business experience might well have had the idea of several rich families investing jointly in hiring the services of particularly expert teachers: wizards, members of the Fraternity of Order, educated priests, etc.
Only about a jillion. :^) The Planewalkers' Guild supports planewalkers. The Fraternity of Order records explorations and unusual events. The Society of Sensation makes a philosophical point of exploring new areas and seeking discovery, then coming back to record their findings. The Fraternity is buttoned-up and academic; the Society is loose to the point of hedonism.
Download the campaign setting from our site and read through it thoroughly. As you can see from the above, there is sure to be a wealth of character background you'll find useful. I recommend that your character's early education have been, after the elementary stage, mostly at the hands of members of a faction whose philosophy he has adopted wholeheartedly and now seeks to pursue.