Check out www.volvotuning.com
for links to companies that make chips for the 850. You won't gain as much as a turbo model, however. A 2.5" free flow exhaust will also help. If IPD makes sway bars for a '93 850, they will improve the handling quite a bit.
If I had a black 850, I'd tint the windows to about 30-35% (not too dark) and lower it a little. For wheels, you've got the oddball 4x108mm bolt pattern that no other year of 850 used, so you can't use any of the 17" 850 wheels. That may also limit your selection in the aftermarket. Depending on what you're willing to spend and what you like, there are some BBS wheels that would look very nice on a black 850 (RC, RX, RK, etc). I think a multi-spoke wheel like the BBS RX would look great in either chrome or anthracite w/polished lip. Of course it all depends on what YOU want, but remember that 17 & 18 inch wheels can hurt performance if there's a great enough increase in unsprung weight.
My favorite look for taillights is half red/half tinted, but it's definitely a personal touch. You could also add some clear side markers and tint those too.
If you want some more ideas, check out www.volvonet.org/tpc
and go to the "other rides", "galleries", and "meetings" sections. Under the galleries section, you'll find the best pictures in the "Vallåkra Träffen" sections (major annual Volvo car show in Sweden).