You can start with one and then do two later. There's a bit more variety with two, obviously, but starting off with one could get you used to doing kbell work.
One thing to add to what Nate just posted, I started with a 36lber and moved to a 53lber within a few weeks. Two of a different size might be something to think about.
Also, if you go with a Dragondoor kbell, check to see if there's any workshops or instructors in the area. Any RKC certified by Dragondoor gets a 10% discount on orders and you might be able to put one in through them.