Sweet build!
Just thought I'd chime in here with some of my own experience. If you're still looking at doing an AIO for it, I've got some advice.
I've been playing with "AIO" style setups on retro PCs lately, on everything from Socket 3 up to 478, including Slot 1 (not 2 yet). It can actually be done incredibly cheap when compared to the cost of a new branded AIO such as Corsair. The catch is that it requires some minor watercooling knowledge (or research), a willingness to sacrifice an old cooler or some fabrication enthusiasm.
Used server AIO standalone pump/cold plate units can be sourced for under $10 AUD, mainly from CoolIT/Asetek. They're great since they are compact and meant for 1-2U chassis. Since they are standalone, they are also perfect for this sort of use case.
All you'd need is a radiator size of your choosing, a coolant mix, some tubing and a few fittings. The difficult part is filling the unit, but it's not an insurmountable challenge if you spend a few extra dollars on additional fittings. I've had good success with EPDM tubing & Mayhems XT-1 Nuke. Of course if you want coloured/UV reactive coolant, you can use some clear tubing too.
Putting the loop together is fairly basic, and only requires one 3 pin fan header for power, though it'd best be served when plugged in with a molex adapter.
Mounting can be done in two ways. The first is similar to what I did with my Pentium III and its cold plate, which is simply using a normal CPU cooler mounting clip and adjusting it to fit for the heightened block. It works well, and mine has been held in place for over a year without issues. The second way is fabricating a bracket. This is what I've been thinking about the most. Using 0.3 - 0.5mm stainless steel sheets works great. You can cut it with hand shears and file it down. As a lot of these AIO units have mounting points built in, it's easy to cut one out to fit its intended shape.
All in all, total cost is around $50 AUD if you need to buy tools and want to fab. Less if you have tools already.
Now you can buy these units already assembled, but it's not as fun and not as flexible. Doing it yourself gives you two major advantages:
- Radiator size of your choosing. You can of course pick a 120mm or 140mm fan for a modern case, but it allows you to mount inside old cases too with 80, 72, 60 or even 40mm radiators.
- Tubing can be cut to length. Great if you need to put it in an odd position.
You also get the benefit of knowing you made something cool (no pun intended).