Hmmmm, 5K really isn't that much in the overpriced BMW world. I guess it just depends on what you are looking for first. You could do brakes and full suspension with a rollcage if you want and tear the hell out of some backroads for the time being. Or you might be able to swing an S52 swap and be fast as balls. It's really up to you. I'll think of something cool though, give me a few hours.