My biased opinion (I own one) would be a '90 -'91 Aprilia Tuareg 600 Wind. Check pix on Bikez.com or check e-bay, they show up from time to time.
Tank size: 17 l including reserve.
Weight: 165 kg dry.
Pros: cheap second hand. high specs (usd forks, dual disc front Brembos and single disc at the back, kick & electric start). Good compromise between road and off road capabilities (fine at both, great at none).
Cons: they were replaced by the Pegaso around 92 and not many were made so probably not many good used buys around. Parts are available from Aprilia specialists except for bodywork in Europe. Parts are not much available elsewhere.
Reliability: not much info on this area, I've only put 500 miles on mine so far from a relatively low mileage when bought. I've replaced kickstarter, reg / rectifier and battery which were already dead when I bought it and serviced carbs and valve clearances and it runs very smoothly so far. I use Metzeler Enduro 3 Saharas which work OK for Lagos's mix of tarmac with potholes, packed sand, gravel and flooded roads.
There's a report from the guy in adventure rider who bought one to have fun off-road.
Hope it helps and g'luck.
[This message has been edited by lepium (edited 25 October 2005).]