Seats shouldn't need re-cutting, I've never had one damaged when motor came in with bent valves from a bad re-build. (snapped valve is a different story though
)
It would be a very good idea to get a decent magnifier and check valve guides though as they can crack when valve bends.
There are also other things you can do to keep costs down.
Instead of having seats cut or lappi ng the hell out of them,get some carborundum cloth tape (the red/brown stuff)
Cut a piece large enough to cover vale head, make a hole in it for stem and break any glaze on valve seat.
Don't mess with valve head, the coating stays on there.
After re-build, set valve clearane to 0.004" intake and 0.006" exhaust. test ride at least 25 miles.
Let motor cool down 6~7 hrs and re-set valves to standard clearance (0.002" intake, 0.003" exhaust)
The idea of extra wide valve adjustment, is to peen the valve and seat sealing up the 'pores'.
It's worked for me many many times in the last 35+ yrs 8)
If you can't find 650 valves, check part numbers for 500f and 550f. I think they are the same but maybe smaller head diameter? (it's just too long ago I was doing this stuff)
You can probably getaftermarket one piece stainless for roughly the same cost as stock Honda
I know this sucks, but, I've never had much success re-using Honda 4 head gaskets.
Even if they initially seal, they stat leaking after a few thousand miles. The CB750 is such a PITA I wouldn't even consider re-using gasket. 500/550/650 can be done with motor in frame so you don't have much to loose trying it. Maybe get some aerosol high temp sealer and spray both surfaces? (I use Coppercote with solid copper gaskes, works for me ;D )