Welcome to the forum.
The TDM900 is a very solid motorcycle and is well designed. It's EFI so doesn't have some of the fuelling issues of late 90s big Yamahas (The TDM850 is a bit mixed on this point).
Obviously the clearances need checking every 25,000 miles - it's not bad job though. The camchains are not an issue, unless big miles are covered, as with all large twins and singles they do stretch on bikes over 65,000miles.
If I was buying one (I have owned an 850, but not a 900, but have owned 7 post 2000 yams) I'd check suspension condition and function, exhaust condition and of course service record. The 900 benefits from a straight forward modification to cold-start system, i'd take look on carpe-tdm for more info on this mod.
All TDMs are brilliant motorcycles, the post 96 850s and later 900s with 270degree firing order are sublime.
Let us know how you get on.