Yow, that's one rusty old Briggs! Looks like a keyed shaft instead of threaded, which is good...
I'd absolutely want to see it run, if it's good all it should need is a jerry-rigged fuel tank and a pull.
Then I'd bring along my compression tester, once it warmed up I'd pull the sparkplug and get a reading - anything over 75-80psi I could live with.
Stray thoughts: Rope starter engines can wear your arm out if they're acting up, but I'd bet you could upgrade to a recoil starter...undoubtedly it's old enough to use points ignition, which means you're going to want to pull the blower housing and flywheel to at least check and probably replace the points and condenser...and that carb is going to need a GOOD cleaning and likely a rebuild kit (aka new gaskets)...and there's just no telling about the condition of the piston rings, cylinder walls, valves, etc until you do the above and get it running...
IMHO, unless it truly does start in 3-5 pulls, idles and runs up smoothly, and has passable compression; $50 is high. If it does all that, then $50 is a fair deal.