Well not thoughts on the adventure module(s) but really on whether you think it would be something that would work for a group not really familar with Planescape and to some extend D&D 3rd edition in general. I am kinda torn but I am biased because I love the adventure and I have been roleplaying a very long time...
Thoughts on Dead Gods/Great Modron March for new players
I would be a little hesitant to throw newbie players into something as convoluted as The Great Modron March and Dead Gods even though that's pretty much what I did with my players. Difference was that they were all experienced 3E players and eager to play in Planescape. If your players are not convinced about the greatness of PS, or totally green, or unwilling to play fairly altruistic PCs, it might become a chore very quickly. My game took almost three years to finish, albeit with many side quests and tangential adventures.
If they're unsure about playing a long-term campaign you should try something shorter, like the Rose adventure in the PSCS or maybe The Eternal Boundary, though the latter has a political complexity that newbie players may find daunting. My advice is: don't be too ambitious at first. You may love Planescape, but others may not, or may require a gradual easing into it. Start slowly, and watch out for motivation. As Southern Oracle said, getting the players to care is one of the pitfalls of both TGMM and Dead Gods. They're both great adventures though, so if your players like PS then you should definitely run these two mega-adventures at some point.
As an overarching plot, The Great Modron March and Dead Gods is a good framework for any planar campaign. The March in particular covers a large amount of time, and not all of the waypoints are developed, so you have the freedom to customize encounters and introduce other adventures to keep the players interested and help the heroes advance in levels. It's also a good excuse for the newbie heroes to talk to a lot of planar folk about the March, and thereby garner information about the campaign setting for the players.
I'm currently running The Great Modron March and plan to go into Dead Gods myself, so I second your choice. Here are some of the pitfalls I've encountered, so you can plan ahead:
The heroes don't care: In some scenarios, the heroes are protecting natives from the March, and in others, they are defending the modrons against attackers. There has to be some motivation for them to want to chaperone the modrons on their unscheduled journey. For some, curiosity is enough, but for others, it might need to be a quest or orders from a superior.
Go straight to the top: When the heroes learn of the unscheduled March, they might want to go straight to Mechanus and speak to the modron upper echelon (or even Primus itself!). Whatever you do to thwart this, remember the first point...the heroes have to want to help the modrons at some point in the future.
Destroy the Tacharim, destroy the threat: This is very intuitive on the players' part, but you can't let them make too much headway early on in the adventure. The clash with the Tacharim is one of the scripted encounters, and it takes place at a certain point during the March, so you can't let the heroes launch a full-on assault on the Tacharim after the Rendering Works incident...keep the Tacharim base a secret as long as possible to delay the heroes jumping ahead of the timeline.
This takes forever!: Make sure you plan supplementary encounters along the March or weave in shorter adventures to keep the players interested. Of course, you can ignore the travel rules for the Outlands and make the March take significantly less time, as well.