I'm looking into buying an 840ci, automatic, 90k miles on it, I just want to know if insurance is particularly high and if maintenance is also expensive
It's selling for 10 grand but I'd be able to invest in repairs not including insurance for up to 20k, is it still an unhealthy deal?

Investing implis you will get a return. If this is the car you have always wanted and you are prepared to basically piss away $30 grand then go for it. Just keep in mind you will never see the money again. If your ok with that then theres no reason not to. The E31 is still a great looking car but may cost the 20k to get back to tip top condition plus a grand or more a year to keep it there. I work on BMWs so i get discount part prices as well as do the work myself. I still wouldn't get one.

the e31 is high maintenance and costly if you need to buy spares for it(if you can find them).