Well i would hope that the XP you are getting already has SP2 with it. If it was made anytime after June of last year it should. But as ibb said i would go with New Egg. I have dealt with them many times and never had any issues getting a issue resolved.
As for your question about the chances of getting a Dell or HP OEM disc, i dont know. It all depends on where they purchased the OEM discs from. If they got them from M$ then they wouldnt be. But you never know. Dell is one of the biggest manufacturers of PC's so it is possible. Best bet is to email and ask just to make sure.
I do not accept support questions via EMail, PM, IM or my G+ page!
Phone: LG Optimus G Pro
Running: Stock JB from LG with Nova Launcher