Don't be too concerned about a little play between the slide and the frame.
As Old Fuff has explained, the Hi-Power is a service pistol, and designed and intended for some pretty nasty conditions. It'll keep right on ticking when a lot of tighter guns have given up and quit working.
And as for worrying about accuracy, remember that as long as the barrel and sights (slide) line up in the same way for each shot, the pistol can and probably will be quite accurate, even though it might still rattle a bit when there's no mag in it and the hammer's back.
Anyway, go shoot your new HP, then come back and tell us what you think. Personally, I'm betting you'll be pleased, and forget all about that little bit of looseness in the slide.
J.C.