Welcome! :) Interesting history, there; I played MM2 back when it was new, but haven't in years :( (and was too early for the whole modding scene). Alarm fur Cobra was cool, too, and I got to try some of the mods for that. But Racer! Haha, I loved that one! It was such a niche game, and I never got the H-shifter to work, but I loved it anyway. :) Great so see someone else who's heard of it.
As far as entering the TDU world, there is, unfortunately, very little documentation. One of the modders, Tool831, created a brief intro to importing a car that could be useful. Before you start, though, you'll need Djey's TDU Modding Tools, which allows you to open and extract from TDU's annoying .bnk files. You might also need to contact an experienced modder, but be warned - some of them are less than pleasant and short-tempered. I'm sure you can find someone to answer your questions, though. :)
List of stuff:
Brief Tutorial (Tool831)
Djey's Modding Tools
1.68B "Community Patch" (This is not strictly necessary, but it helps out)
Before you can do anything with changing files, you will need something called a "magic map" (which you may already have, depending on whether you've used mods yet), which lifts the size restrictions of the .BNK files. But that should come with several of the above automatically. :)
Happy modding and good luck! :)
-Leadfoot