Skeet and HD are the two ends of the continuum as far as barrel length goes. HD you want short 20" to 24". Skeet, trap, and sporting clays you will tend want to the long barrels 28" or so. The 870 platform is nice because you will get the capability to change out barrels easily. $100-$150 per replacement barrel and you can swap to full rifled(Sabot slugs), Ultra full(turkey loads), short with IC choke(good HD). Realistically, a 28" can work for both, but it will be less maneuverable in a HD situation.
As far as mag count goes, for skeet all you will need is 2. The standard model should be fine, I belive it is around 5+1. In HD you are not likely to clear 8 or 9 rounds a riot gun holds before the BGs are down or fleeing.
All in all, a 28" with standard magazine (extensions can be added if you want them) should fit the bill. Removable choke tubes are a nice option that help you get the most out of your purchase without buying or swapping barrels.
I am a Mossberg Man myself, just personal preference the 870 is a sturdy and reliable gun too. Check the slide for wiggle and excessive wear it should be smooth and easy. Watch signs of rust and rebluing. Original finish is best. I know the college crunch I am just getting over it myself. Shop around, you should be able to find a good deal. There are a lot of 870's available.
Good luck with your purchase!