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Unless you have a managed server, you'll be faced with stuff like upgrading the server software from time to time (not cheap with Microsoft), generally maintaining it and theres also the problem that the server itself will need upgrading eventually. Do not underestimate the costs involved in going that route - or indeed the effects it'll have on your social life. Its a millstone from hell.
Cheapest option and with the least amount of worry is to get your own IP address.