Hey, I had a vary similar problem with an 86 745 i bought used. The po told me and I saw that you had to come to a complete stop in order to shift into first. Maybe a month later I was having really bad problems, like having to double clutch to get into second, or not even being able to get into first or second at all. So, I bought a used tranny and replaced it. You seem to be adept at fixing stuff on your car already. I don't think the mechanic will tell you anything different from, you need a rebuild for $800+ or a used tranny for 200-300. I chose to buy a used one and put it in myself. I had less experience than you (you've already adjusted clutch and changed fluid) and the job wasn't too bad.
Good luck with whatever you choose,
Nathan