mariner = mercury
Mercury/Mariner is really just like Evinrude/Johnson or Chevy/GM/GMC - same thing. Some people like GMC, but won't touch a Chevy. Anyway, sometimes they have made the same engine and labeled it as a Mariner & a Mercury; other times they only labled it as Merc. or Mariner. If there's was a Mariner 115 made that year & a Merc, they are the same engine. It's all marketing. Mariner has a tougher logo to make you think it's a workhorse. My .02.. Mercury/Mariner is OK and everything; with used engines, the #1 objective is to get one that's in good condition, regardless of brand (except Force - another story). I probably would not buy a new merc.. have not been very impressed with them in general, if I buy a new OB, it's probably going to be OMC (Bombardier)... if one dropped in my lap, for the right price, I would certainly pick it up though - pleanty of happy Merc. owners out there.
Anyway, outboards - #1 thing is compression, typical readings depend on exactly what engine (between 100-150).. most important thing is it should be even - no more than 10 difference from highest to lowest. Check the gear lube, it should be clean; milky white = water, black = old/burnt or it got too low, chunks in it - very bad news.. little hairlike splinters on the drain plug magnet is normal & OK. Have him leave it cold and let you start it yourself - do compression check when it's warm. Take it out for a drive, if possible, or at least run it for a while, observe good water flow and be sure it's not overheating or anythig.. check for obvious flaws & excessive corrosion etc.
As for maintenance records.. sorta a plus, but don't make any decision on those.. most maintainence is something that the owner takes care of (ie grease etc..) an occasional tune-up or water pump impeller change is great too, but not usually neccesary. The current condition of the engine is everything, work that has been done, with the exception possibly of a rebuilt power head, does not add value.
As for a 50 HP outboard, are you talking new,used? 4 or 2 stroke & what make.. The prices fluctuate quite a bit w/ the 2, 4 strokes & make. A good deal on a new leftover mercury might be $3500... a not so good deal on a honda 4-stroke might be $6000 - somewhere in between there. A couple things I'd like to add - in this HP range, you can get away with using a 4-stroke without adding much weight. The Johnson 50 is about the lightest at around 180 (2-stroke).. a couple of the 4-strokes are around 250 at the heaviest. The downside is that a 4-stroke won't have quite the same kick and will run you some extra $$. As for used, you could pick up something reliable in that range (ie want adds) for as little as $1000 up to the price of new. Good luck,