Or you can buy one of those little ethernet-to-usb printer sharing boxes so the mac doesn't have to be left on. I've had mediocre experience with them (tricky to configure and keep them working but you're going to be paying someone to do that for you).
The Apple Airport Express might be handy for this. It can join an ethernet or wireless network, shares the printer out via bonjour, and comes with Windows bonjour software. No IP configuration at all for the printer, and no worrying about changing IPs later.
The tricky and time-consuming part is going to be downgrading that PC to windows XP (specifically getting all the hardware drivers working properly). You might be paying someone the 100.00 for that alone, and another day's session for all the other stuff.
Unfortunately Doug already has decent hardware otherwise I'd recommend replacing it with a Mac. Even as a Windows machine. I just reloaded my laptop recently, and installing Windows along with all the drivers was painless with the Leopard disc. It really reminds me of the nice Windows installs that were possible on Gateway machines in the late 90s. Automated driver installs of everything, clean install off a stock Windows disk and no extra crap.
While it doesn't address the application and user data process, it's at least a quick start.