SX-780's are solid little units, and they're not too difficult to work on. They are at the point age-wise where gremlins are likely to surface, however, and if you're not handy with a soldering iron you might be better served by upgrading to a new unit. I'd recommend a good midrange surround receiver, they're perfecty fine in 2-channel mode and you have options for future expansion.
I'm a fan of the vintage units, I have an SX-780 that I restored and I love it. Properly refurbished they can give many more years of service. However, paying someone else to do the work can get very expensive. OTOH it's a fine unit to learn techniques on, Pioneer sold a boatload of them so parts units are easy to come by. I have one or two parts carcasses laying around myself. Should you choose to refurb the 780 you'll find it a good match for most modern speakers, and it will happily drive a 4-ohm load too. Common issues are the usual dirty pots and switches, cold solder joints developing under the regulator transistors (easy fix), and occasionally an output module will die. Those are easy to replace if you're handy with a soldering iron, and were still pretty cheap last I looked (I paid $6 or so each for the last ones I bought, the 780 has one per channel.)