The 850 is marginally more responsive and definitely more thirsty than the 840. As a novelty it's nice to own a V12 Bimmer.
Take the time to do your research. Come spring/summer time you'll see more e31s for sale. At the same time you could also take advantage when "GTA 8ers" get together. There you can ask questions and take a glimpse of what you're getting in to.
2000 540i Sport 6-Speed Black & Tan, 18" OEM M-Pars
2000 740i Sport Black All Around, 20" OEM Style 32s
1991 850i Platinum Gray / Oyster, 18" OEM M-Pars
1995 840Ci Hellrot / Black, 17" OEM Style 5s