The Harley Sportster is a truely great bike.
Get one from 1986-1994. 1986 was the first year for the Evo motor and after 1994 the electronics get exponentially more complicated every year newer. Ones in that year range sell fora bout $2000 in my area, which is pretty cheap but about 2x as much as a 1980s UJM...
Pre-1992 will be a 4 speed and post-1992 will be a 5 speed. Personally, I prefer the 4 speed since the final drive ratio in top gear is the same... so your highway RPMs are the same with a 4 or 5 speed. It has plenty of torque, so I prefer the less shifting and simpler tranny mechanics of the 4 speed. Its bullet proof.
Its a Harley, so cheap parts are everywhere. It only has one carb, which hangs off the side for easy access. It also has hydraulic lifters, so younever have to adjusta valve. Older ones are chain drive, which I prefer, and easy to maintain.
They have very reliable alternators, solid state regulators, and electrinic ignitions that last forever. Did I mention how cheap and easy parts are to get? I have seen centerstand kits for the Evo Sportster, but the factory only offered a centerstand on sportsters from 1957-1978.
The early Evo sportsters are as simple, bullet proof, and easy to maintain as it gets. Plus, they are a ton of fun.
Get some taller shocks and knobby tires and they do ok off road too!
I also love the xs650. My only problem with the xs650 is that their stock charging systems suck and are very expensive to get parts for. Even one in good shape will barely charge your battery below 2,500 rpm.... so do not sit in a traffic jam for too long. Newer perminant magnent alternators can be easily adapted or bought as a kit and you will have a very solid bike. A nice XS650 will run you about as much as an older Evo sportster, about $2000 in my area. Stay away from really beat ones. After you fix the carbs and charging system you will have spent more money than if you bought a nice one to start with.
The yamaha xt500 is about as simple as it gets, and tons of fun... but might not make enough power for you. I weigh 150 lbs and can comfortable cruise on my 1978 xt500 at 65 mph with a -2 tooth rear sprocket installed. Again, these tend to cost more than the average UJM... but I guess its expected that you will pay a bit more for true greatness...