I would ask myself, ...
Do I like working on mechanical?
Do I have the skills/tools or want to develop them?
How much mowing will you be doing?
If the mower fails does that put you in a bind?
Can you actually find parts (blades, plugs, cables that fit this mower?)
Shop / repair labor $$ are high and if you have to take it to a mechanic one time then the ownership costs are going to go way up or they will not work on it due to the age.
IMO, Liking retro is one thing but for me that really fails into a catagory I would call "a hobby". The old stuff is built way better than the over-price crud at BBox stores but it requires more attention. Small things like cables, wheels, carburators, rusted decks, blades can cost you some time and some coin.
I own a mid 80's Honda Harmony II "first owner" that still runs and operates perfectly, always cleaned and kept inside. Oil changes and such every spring. I like the plastic deck as it does not rust. Anyway, I was a broke lad back then with two kids and I paid probably $600 for that mower. Was best decision I made back then. Still have and use all the time.
Just so you know, I love old gas stuff. I still own;
a mid 80's Stihl 031AV 20" chainsaw that is still my go to saw for firewood cutting. I am building a MS390 though.
an old Honda ES6500 generator that is still my home standby power for blackouts.
a big ol Power Trim gas edger that is old and perfect.
IMO, the seller of that mower has something that will be very hard to sell. Have some fun, go get it for $10 -$25 dollars and give it a whirl. It will probably need some work. Even though the carb, gas tank, gas lines, filters will need some attention. I would not buy anything until you actually troubleshoot any problems it may have or see obvious issues to resolve.. Part availability will be an problem. Enjoy your retro mower till you don't.
Well, that is too long a reply, enjoy...