I was also rather fond of the Honda XL350/500 singles, and the Yamaha XT/SR500 from the 80's, but I'd definitely want better brakes than what they had. I suppose a modern KLR650 would get you liquid cooling for hot climates, the power to handle a sidecar with extra gas & parts, and the ability to handle highway speeds when the occasion arose without getting run over. The KLR250 would be a very good solo bike, but I don't know if I'd want to ride it on a 55 mph highway with a sidecar - the car would weigh as much as the bike - very unlikely to tip over but a passing truck would probably blow you off the road.