Can only get one disc? Well, that would be the most used disc in the game, a putter. So, I'd get a good all-around putter like a Wizard, Aviar, Challenger, Magnet, Magic, Reef/APX, Voodoo, Ridge, Ion, Summit, VP, etc. But all those are best in baseline plastic and beginners hit lots and lots of trees so premium plastic would be better. I don't like the overstability and lack of glide of a VP or even a Ridge so my answer is:
Ion or Summit.
Loan disc? I usually toss a Polecat at beginners but I wish I had a Z Comet solely for this reason.