We got the 300 set. It seems great and my son loves it! It seems like it will keep him busy for awhile. We typically do 3-5 a time - usually daddy does them with him because he feels left out if we do them without him, lol.
They do make add on sets that 'upgrade' you from one set to another so it's not that big a deal to get a smaller set and then need more. I think I would go with at least the 300 though if you can get a good deal on it. We got ours for cheap from Amazon near Christmas and I'm very happy with it.
