Well, i don't have an 85, but... i believe all radios are the same size. There are two flavors. There is the single height kind and the double height kind. I don't know what exactly they're called but that's basically how you can think of them. You should have the single height one. Any standard thing from best buy should fit in there without too much fiddling with it. Of course to get it to look nice you might have to mess around with it. They're all different.
I have a sony head. It's nothing too special, it's pretty old now too. It' just a cd player with a removable face plate. It has a sub out, so the subwoofer is controlled seperatley from the main volume. It's a nice feature to get when looking at stereos. The outputs on the back are just plain RCA - phono jacks for the sub. It also built in crossover where you can tell it to only send the low frequencies to the sub instead of everything including the highs.
If you're going to be buying new speakers, just match the speakers wattage to the head unit you are looking at. I have three way speakers in my truck. Mids, tweeters, and super tweeters. I listen to alot of classical music so wanted alot of high treble end. They also make 4 way speakers with another super tweeter on there. But they didn't make them in the size for my truck.
For a subwoofer, talk to argve, he probably has a good idea of where to point you. You'll i guess need something to fit behind your seat, so there probably isn't too many options for you, but i'm sure there are speacial ones made for pickups.