I don't know how Shuttle's power supply calculator works, but I'd highly recommend some headroom in that area. First off I don't understand how that kind of machine could ever run on 242W, but even if it will, you're giving yourself no room to expand at all.
$300 for a case/PS/mobo doesn't seem that such a killer deal that you have to use this either. My last custom build was in an
Antec Fusion case (430W quiet PS) that looks to have more features than the Shuttle. I don't recall exact prices but case & mobo was definitely under $300.