Britax does fit. Legroom is tight if you have a large passenger, but not a problem if you have the freedom to slide the seat up a little. There's a bit more lateral play than in an E46. Funny enough but getting in and out of the E30 coupe is actually easier for a toddler than with an E46 sedan... the folding action of the front seat provides plenty of access room.
LMK if you have more specific questions.
kurt