This is just my opinion, but if you are going to go through the process of building the two power supplies, spend a little more money and get a more powerful Tripath board. 20 watts is pretty wimpy unless you're committed to building a system around high efficiency speakers.
The 5V supply will need to regulated and grounded separately from the 25V supply. Others on AC know more about this than I do, but I doubt you can buy an off-the shelf PS solution to run this board.
My advice: if you want to keep it simple, stick with one of the 12V Tripath boards which you can run off readily available stand-alone power supplies. Otherwise, be prepared to build (as in buy parts and solder them together) two separate power supplies. I've done it and I'm willing to share what I've learned. I just want you to know what you're getting into.
