"a well sorted car from the get go that is ready to drive."
Tim,
Sounds like expectations are realistic. Somewhere between $25k-$45K should land you a driver that people will step back from and think, "wow", that is one nice car. Plus, you will be able to drive it from point a to point b and not worry about it. Go for it, wait for the right one. bgl It should be a blast.
I actually think the "well sorted" part will be a bit of a challenge. These are 40 year old cars so they will have lots of worn out or age-degraded systems on them.
To maximize your chances in this area, I'd focus on 3 things
(1) try to find a car that is ALREADY being used as you intend. Cars like Stan's, Adawil's are examples of regularly driven cars that are well sorted/debugged . A restoration with 500 miles on it is not likely to be as good as a car that was restored 5 years ago and has 15k miles on it.
(2) Make sure the car has many years of documented maintenance/service from a shop with expertise in 1970's era BMW's. Its amazing how many broken bits us hack mechanics (sorry Rob) will put up with because we "know we could fix it if we wanted to..."
(3) Have a PPI done by such a real professional with experience on these older cars.
3 years ago I sold a car that I think meets your requirements (20 year old restoration, everything worked, no rust, great documentation, professionally debugged, showing a bit of patina, a 10 footer wow) for just over $40k. It sold without ever being advertised to the 1st interested party. You didn't mention if the exterior needs to be a wow or if the required interior condition. If yes, you are probably closer to the $40k range. What about a really really nice 2002tii with Behr AC? You could get one of those for around $30k and I think there's still real upside left on cars like that.
For $30k, you might have to accept some minor rust issues (rust covered up rather than actually removed).