Of the 10 vintage receivers that I have bought, I sold one, gave another way, continue to use one as-is, have another two in the queue for eventual repair, and I refurbished the other five.
All five needed the work, and sounded much better after getting it.
That receiver does not look like some pampered cream puff.
I would expect it to need extensive work, and then hope for a pleasant surprise.
Having someone do the work for you is not a viable option if you weigh repair costs against post-rebuild resale value.
The 780 (and most of the x80-series) has integrated power amplifiers that must be replaced as units, and are difficult to find.
I would not buy this receiver without carrying a set of small speakers / headphones with me to assure myself that both power amps are working properly.
Without clean sound on both channels I would walk away and find another, nicer unit in excellent condition.
1970s Pioneer receivers are not hard to find.
I bet a WTB post to Barter Town will turn one up very quickly.
Earlier Pioneers did not rely on the integrated power amps.
The SX-x50 series look very similar, but without the black power meters.
HiFi engine has docs:
https://www.hifiengine.com/manual_library/pioneer/sx-780.shtml