My advice is seriously to stick with that '95 LS. That's an excellent car and has more HP (200) than a 528 (about 184). As you know, the Legend GS has 230 HP.
Your Legend is the match of the E39 in interior appointments, quality, and most ergonomic aspects except the BMW's seats are far more sporting/supportive. The power is similar and the smooth ride will be comparable. However, the Legend's handling won't be on par with a 528. Its longer wheelbase cuts nimbleness, and the roll stiffness can't compare to the 5-er. There isn't much aftermarket support for upgrades either.
If you must get into a BMW now, I'd recommend an E46 3-series, or an E36 3-series or E34 5-series. They are definitely more reliable and cheaper to keep at those higher-mileage levels (above 70k) than the E39. And still very rewarding to drive!
As for our Legend GS, which we owned from 101K to 189K miles, the only things that "broke" in that time were a leaking pinion shaft seal and a broken rear transmission mount. THe shaft seal is a common '91-'95 Legend malady around 100k miles - a $5 part that required $425 labor to replace. But that's about it.....
To answer your Q - the 528 would be somewhat cheaper to maintain, but not enough to matter (radiator issues are fewer in the I-6 models). The Sport suspension increases tire costs since the staggered tires of the sport package cost more and could not be rotated. So a non-sport would save some coin. But many of the same problems are common to both 528 and 540: expensive suspension bits, relays, brakes, wheel bearings, air conditioning, p/s fluid leaks, dash minder pixels going out, cam sensors, etc. etc.
2006 Honda Odyssey EXL-RES
2006 Honda Accord V6 EX-L Sedan
1999 BMW 540i 6-Speed (Sold to M3UOND on 9/21/06)
1991 Honda Civic Si - Occasional Beater
Last edited by Eric S.; 08-17-2007 at 08:37 PM.