My cutters are going to go to the Clockwork Nirvana, heading for the Fortress of Disciplined Order in order to get some special information.
Their way will offer lots of opportunity to Climb, Jump and Balance to get from gear to gear, they will come along a city of petitioners (or who else is supposed to live on an outer plane) on a fortified cog that is besieged by expanding formians ('north-eastern' part on the map...) and maybe explore an abandoned monastary until they reach the Fortress.
My questions:
Am I missing some 'must-sees' of Mechanus? 
Strongly Lawful aligned. That means someone not of the plane's alignment gets the penalties described in the DMG: -2 if one part of the alignment is differing (like NG) and -4 if both are (like CE).
Do N cutters get -2 or -4? (Please correct me, when I am wrong)
How do petitioners work? Just adding the petitioner template (http://www.systemreferencedocuments.org/35/sovelior_sage/divineMinions.html) to normal NPCs?
Are there really hostile creatures on Mechanus? Folks like Inevitables, Zenythri (SP?) or Modrons don't seem very dangerous, as they probably won't start hostilities with adventurers themselves. Formians (at least in the situation my players will meet them) seem to be the only exception; the monsters in the Planar Handbook and the DMG I either don't have, or the are from the upper or lower planes... So what wandering monsters has a basher to expect?
What about landscape? The cogs & gears of mechanus are of metals and different kinds of stone. Some of the larger ones have 'normal' landscapes by Material terms with hills & water, plants & animals, and also house the settlements. Or am I wrong?
Edit: Fixed the planar alignment mistake
Or have someplace they're getting to only be accessible between the gear cogs. Or even more evil... the portal out only activates two seconds before full crush - and they have to hope they're in the right spot when it does... 
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It's Neutral aligned on the Good/Evil Axis, but Mechanus is DEFINITELY Strong Law aligned. That means cutters who are neutral on the Law/Chaos axis will receive a -2 on all CHA-based checks, and Chaotic ones will receive a -2 to all INT, WIS, and CHA checks. Good and Evil types won't necessarily be affected, as the effects are only based on their ethical alignment.
This will hamper a wide variety of casters, as well as some rogue-types.
Creatures one might meet are Axiomatic versions of creatures found elsewhere, as well as the aforementioned modrons, formians, inevitables, and zenythri. Also common are Parai, which are LN exemplars from Mechanus, but tend to take what they want forcefully, and they like beautiful things, as they see beauty as being a form of perfection. (Honestly, Parai, which are known by another name in 2nd ed. AD&D, are a good foil for the Modrons and Formians)
Modrons may not seem like they are likely to be particularly hostile, but they are exemplars, and exemplars of Law, which means they're gonna act in ways that may seem bizarre to the irrational cutters from elsewhere. Appearances can be deceiving; while their appearance may make them seem harmless, they are not, and they can be as likely to take offense as any. The problem is that when you've made one mad, (unless that one happens to be rogue) you've quite possibly earned yourself the enmity of an entire race. Same with the formians and inevitables. Chaotic cutters won't be doing well in Mechanus, and it's entirely possible that modrons won't take well to the presence of chaotic cutters... (if your PCs are all LX, or NX that's less of a problem, though)
Inevitables are more easily dealt with; their main concerns are pretty easy to figure out. In order to avoid the wrath of inevitables which deal with people who break contracts, don't break contracts. The same follows for each of the other classes of inevitable. HOWEVER, if the PCs have done something stupid, then all inevitables of that type will hunt them down mercilessly. Such is the way of the multiverse. In short, they're RELATIVELY harmless, but only when cutters are smart and can manage to stay out of potentially dangerous situations.