Need advice on '77 E21 320i auto

I really, really like it, and it is in just cherry shape, but I live in MN, and the winters are brutal. The clock shows 133,000, but the guy I bought it from says it stopped about 15,000 miles ago, and I believe him. It was a CA car, and there is NO rust. When driving, the only thing that causes me concern is a kind of "clunk" when the auto shifts from 1st to 2nd-it makes me worry that it may cause the universal joints to fail. This doesn't happen all the time, maybe 50%. The thing is, I have maybe enough "car knowledge" to know when something is not right, but not enough to be able to accurately diagnose the cause, and/or the fix.

Oh, and there are some issues with the heater/defroster-it puts out good heat, but when it is REALLY humid or rainy, the defroster first makes the windshield worse, before it gets better. But I can live with that, for awhile.

I know most of you are into newer cars, but I wonder if I should keep it or sell it? I would really like to install a manual transmission into it (rather, have a mechanic install it), and, other than that, keep it pretty much stock (other than replace the ugly front bumper!).

Bottom line, I suppose, is that I feel like I have stumbled into a real classic, in just the sweetest condition, and I don't want to f**k it up!

??

The trans could probably use a service, like a fluid and filter replacement. On the bright side, at least it doesn't clunk all the time. If you want to retain value, you should probably investigate the effect on value of installing a manual trans. If it makes no difference, go for it. Kinda improves the driving experiance i think.