I remember seeing a headline on BBC a couple of years back saying "Cleveland Taylor joins Saints" and panicking, thinking "this is literally the worst thing we've ever done" until I clicked and saw St Johnstone.
If it was only 1 signing and no loans then for me it would have to be a GK. Can you imagine what wold happen to our goal difference if Forecast had to come in because KD picked up a red. We would be -12 quickly. I'd think we really need 3 more loans or permanents. GK, RM and CB. We have options at RM but nobody grabbed the spot while we were in the Championship. I don't see why they would be better able to in the PL. CB depth is now down to 1 assuming Dan S is actually able to play.
If we could only make one signing, it would have to be a quality right midfielder for me. Ideally one with real pace, but I'd have no problem if it's a tricky (ideally two-footed) winger with a top class cross on him instead. But the simply fact is we need more than one player. I think everyone generally accepts that we need to sign - either for the first XI or the bench - a GK, a CB and a RM. And on top of that, there are maybe one or two other positions which would be someone's personal preference to improve.