1986 is the BEST year for the 4 speed Evos.
In 1986 heads on both the 883 and 1100 had bigger valves than any other year.
The shift shaft oil leak will be EASY to fix. There is a seal in the primary cover that the shift shaft passes through. Its a $5 part. You may also need a new primary gasket. Even with no experience wrenching on a Harley, its a 20 minute job, tops.
Sportsters are unit construction (primaty, tranny, and crank case are all 1 piece... well, its 2 actually, but they are pressed together). The tranny and primary share the same space and fluid, so the only place the shift shaft can leak from is the primary shift shaft seal. It is a fairly common failure.
Do not let anyone diss the 4 speeds either. The final drive ratio of the 4 speeds and the 5 speeds (with stock gearing) is basically the same... so your highway RPMs are about the same too. I prefer the 4 speed, these bikes have good low down torque (even the 883's) so less shifting is nice, and they really arent any slower for the lack of 1 gear.
Make sure it charges. The 4 speed sportsters had issues with the magnets falling off of the stator (it was not very common, but it happened enough to get a reputation). If that 1986 charges and runs, buy it. They are as close to maintinance free as a bike of that age can be.