Best engine I ever had till my ETEC 75 had 2 of them one was a Classic 50. (see pic)
As always the critical part is testing compression, look for near equal+/-10% evaluating overall level of TLC received since 1964, (picture shows a really nice well kept engine ) salty or fresh water engine ?. …actual test run result if possble by seller . if auto oiler (added on a 1964) I would get rid of it and go premix 50/1 check wiring for crumbling wiring (possible easy fix if handy and good barganing chip to lower price.)
they do love fuel being carbed but smooth idling all day on 5 gal.
if you buy….do usual stuff like, change impeller check GC oil for leaks try a pressure-vacum test, refill , clean the carbs , check ignition etc… 
and a Mercury Black spray can will make it like new but your’s probably just needs a touch up on the GC !!
Hard to service compared to OMCs Yes to some extent but experts here will help, visit John’s old Mercury site and get a service manual
see cheap and good source to download books
Mercury Outboard Manuals and Models Directory
note: on my first 50 that fat side plug needed surgery due to a broken wire inside due to flexing .. but it was an easy fix ! engine would die when turned to one side. to port if I remember….
Joining AOMCI has priviledges 🙂