RE: Single shot .223
If you think you may end up wanting a bolt-action, my suggestion would be to go ahead and get one. Everybody needs a .223 or similar caliber in their gun cabinet. If you want a semi-auto, you are going to have to shell out some serious bucks for an AR-clone, when you can buy a good bolt-action and equip it with a scope and still have money left over for ammunition.
Whatever you get, be prepared to shell out some $$$ for a decent scope. Optics are very important for varmint hunting, and without them your gun is nothing more than an expensive stick.