My thoughts:

1) Air conditioning, brakes, transmission.
2) Not many '99 V70 T5Ms out there, in my opinion.
3) Since the M3 is the top of the 3 series foodchain, you'd best look for a T5.
4) Most T5s have 16" wheels unless they were upgraded.
5) I have Pilot HX summers and Gislaved winters. I am going to try Kumho summer tires next because they're inexpensive. Brakes are pretty reasonable. Check fcpgrotonllc, ipd, volvoparts, newandusedvolvoparts, etc.
6) There is the euro version of the Haynes manual for $27 or so from ipd. I still use my 850 Haynes manual.
7) I don't know that there's any difference in parts quality between a '98 and a '99.
8) Don't know about trade-in value. I have owned a previously leased S70 T5A for about a year now.

Good luck in your search.