It will work alright, but there are a variety of these adapters around, for instance on ebay. But why not put a 3.5" hdd in the machine?
Anyway, for testing the computer, power it up with no harddrive installed, and if you get a purple screen after half a minute, the computer is working. However, that's not meaning you haven't broken anything, but at least it's powering on, and you can go from there.