Can't comment on your Johnson oil dilemma, but I've heard of lots of people removing the VRO system from these motors and pre-mixing the 2-stroke instead. Read into that what you may.
The 3 cyl Suzuki 55 is a cracking motor - basically, a detuned 65, which was one of the leading outboards in its class at the time. Ther's nothing really problematic to look out for, other than the general stuff with all outboards: cooling, compression, gear selection, etc. Some of these didn't have power trim & tilt - not essential, but nice if it's there. Value will depend on its condition and use - I suppose if it's in good nick and running nicely (with PTT) then you should do ok if you get it for less than £800.
What's he asking for it and do you know how many hours it's done