An All-In One computer is one that I would avoid entirely. Most if not all of them were designed for the consumer home market and had very weak CPU's in them, many had Pentium D and AMD E-Series CPU's in them. If you could find one with 1394, it would be very old (5 years plus). I remember HP had one at one time, but it's not worth pursuing.
You'd be better off getting a desktop system either as a refurb and adding a Windows Touch Enabled Monitor.
IMHO, a FireWire enabled computer under $500 would be a refurbished computer like a Dell T3400 workstation which can be gotten on ebay for under hundred dollars with free shipping. Look for one with 8GB of RAM and at least a Core 2 Quad CPU, most systems will have Windows 7 on them. The business world never really adopted Windows 8, so you'd have to load that on your own. You may have to install your own 1394 card, although many T-Series Dell's came with them already equipped. If not refer to our 1394 card compatibility guide to find one that will work for you.
The alternative is to build it yourself. You can still buy motherboards with 1394 on them, however sometimes the onboard 1394 chips were no good unless it was a Texas Instruments chipset which more often than not it was a low quality VIA chip. See the 1394 compatibility list for more information on which chipsets to look out for.
So either refurb or DIY to keep the cost under $500.
I would suggest discussing this topic in the forum with other community members here: https://forums.presonus.com/index.php
Hope that helps.