for the price you cant beat a 83-84 250r. For about $1,000 you can get a nice one. No need to upgrade it because its great the way it is. The 85-86 is really the king of all 250rs but to get a basic nice bike will run you $1200-1700 and with so many big bores and aftermarket HP goodies you will start spending lots of money and soon will have lots of money into it (trust me I know!!!) if you want something just to ride right away and not have to upgrade or redo and not spend a fortune, get a 83-84. If you want something you can fix up and mod and get more and more hp then get a 85-86.
On the carb note flat slide carbs are better but dont think they raise hp that much many throttle response. Besides why worry about the flat vs round on 85-86 when you can buy an upgraded 38 flat for both
85 ATC 250r -Under Construction
86 TRX250r
05 KFX400
02 400EX