Superb bike. I had a 1980 R100 for years. My input is as follows;

Buy big engine. At least a 750. The biggest available is 1000cc (R100). No BMW will keep up with an oriental rocket but the R750/900/100 will cruise effortlessly and unstressed all day and all night.

Buy 1980 or earlier. There are persistent urban legends about post-1980 quality issues. I can't say for sure but I had NO corrective maintenance in the 8 years I owned mine. Only wear items and preventive maintenance. As with old Harleys and old Mercedes, there will aleays be plenty of parts available.

1981 was the first year for electronic ignition, a maintenance advantage. The 1980 was the transition year with the point setup mounted in the same can that the electronic ignition was in the following year. This made breaker point maintenace very easy. Pull the can off and take it to the workbench. A less than one-beer job. Earlier than 1980 the points had to be replaced or set in place on the bike.

Hope this helps.