you'll want anything built before '69, before they went to plunger forks; look for an example with earles forks: if it has earles forks then it has a factory welded set of ball mounts...
if you look at the literature, you'll see that the sidecar for your bike is the LS200 or LS250. small as a munchkin's shoe, but anything bigger would be way too heavy-and there are those of us who feel that even the little LS200 is too much for the bike. but hey: BMW did sell them set up that way...
I have see a really nice set-up that came from the Dauntless folks in Seattle.. it was a F650 using their subframe (somthing like $650) hauling a Russian light weight sidecar.....A sputnik (I think) It was featured on the cover of ON magazine (it was painted Yellow)
I ride a sidecar rig... and this is a really nice rig...best single rig I have seen...Check the Dauntless website.
In 68, you could get both Earls forks, and telescopic. I watched as a dealer took the forks from a telescopic model and replaced them with a pair from an Earls. Might be careful of what you buy if you are buying a 68 thinking it will be Earls. And, even worse, might get a 68 or whatever model with Earls forks that does NOT have sidecar mounts!!
The R27 would make the best single-cylinder sidecar-puller, as it has the most power. However, the 18 hp R27 and 15 hp R26 don't have mounting points, but the early-50's R25 does.
For the R26/27 you would need to buy a lower subframe having the two lower ball mounts, and at least one eye clamp for the upper mount. Some use 1 upper mount for a total of 3, but two upper mounts are better for a total of 4. You must also change the final drive to a higher ratio, which will reduce the top speed to around 45.
I have more info and photos if you can email me.