I drive a 98 S70 and my wife has a 95 850. The reliability
is similar (although problem areas differ a bit). Maintenace
is a bit lower on 99 model year and up. The 70 series rides
smoother and might be a better choice for someone doing hours of
highway driving every day.

The 98 model year power train is basically the same as the 850s.
In 99 they started adding electronic throttles; variable valve timing;
5 speed automatic tranny, coil on each spark plug, etc. So, if he
is used to working on the car the 98 would be close to what he is
used to maintaining. But, 98 was the first year and had some
electrical issues and some other teething problems. A used 98 should have
the bugs worked out and I would check any records for re-occuring problems
if looking at a 98 (well, any year, I guess).