Get a LWB /5 or better, a /6 with the stronger frame. /7's had further reinforced frames. You do not want to use the SWB swingarm. Trust me on this.
The 750/6 was, IMO, one of the best Beemers ever made, really delivered on the "balanced performance" BS. The 750 engine is a sweet one. The later 800's are also very nice, but feel very different to me, not as punchy or highly tuned, but very smooth and revvy.
18/19 tire choices are limited, but they do exist. Problem with the old BMWs is gonna be rear tire width, as Chop said. Cagiva Grand Canyon runs a 19 front, so there are some Pirellis out there, and some nasty old Conti's.