There are pages of full threads on the board about 'what could go wrong', 'probs with E70', 'what to look out for in a used E70', etc. ...many not exact thread titles, but lots of opins posted.
If you haven't read this, it is a good overview... see post #6 by Qsilver7 in this link:
http://www.xoutpost.com/bmw-sav-foru...dering-x5.html
And, another:
http://www.xoutpost.com/bmw-sav-foru...d-options.html
There are also pages of threads on 'auto trans problems'. Some posters have had more than their share of probs, others have gone 100k+ miles prob free. It is hit or miss, but something to be aware of that might go boom, sooner or later.
Some reading on the specific E70 forum should help, too.
The dearth of responses to your post is probably a function of this kind of question coming up on this Juke Box about twice a week, and a check list of potential probs covering all the E70 MYs, options, engine configs, et al, doesn't exist that I know of.
Imo, your later MY picks are probably the right way to go, and lower miles/one owner may help, too.
And, fwiw, there are hundreds of posters' cars on this board that have run for years/hundreds of thousands of miles, with very few probs. My '01 3.0 has had barely a handful of non-main repairs over these 12 years. All anecdotal, but your DIY skills and interest should serve you well.
Do some reading and good luck,
mD