On the contrary, an inline unit would be easiest, and would make the most sense. You will need a decent radiator, res, and pump, but access to the pump for verification of how well it is doing is crucial for this kind of application.
Think about it. You'll have multiple computers on one loop. If your pump goes out, you are f*cked, and if you can't inspect your pump easily, you'll never know if you're about to suffer meltdown.
The res is crucially important. When filling a system, you will need plenty of fluid to act not only to help prime it (this is what I love about my Tower of Power [tm], but you probably don't want to go that route
), but also to serve as a buffer in case of extreme variations in the coolant temperatures. With multiple machines on the loop, you want to be able to normalize the temperatures as much as you can before you get it to the radiator, and a reservoir is the perfect place for all of your systems to drain into, and a perfect place for your pump to prime from.
If you look at the front page of my article I pointed you to earlier, there is a diagram of a way of assembling the system using a submerged pump. It would not be difficult to build a PVC infrastructure to isolate the pump inline with the res and radiators, and with shutoff valves as well, helping isolate your system in case you needed to remove the pump for maintenance (without draining the entire thing ... though my system is easy to drain and fill, I still wish I would have done a better job of isolating the pump for maint.). I would suggest a res of approx. 6L. It would be big enough to prime a good portion of your system without taking too much U space. Just set the tank on a rack shelf. In the space below you could take 8-10U and build a wind-tunnel (the equivalent of the box I made, a completely shrouded off box, essentially, forcing all air from the fans through the radiators in order for it to escape ... works very well) to house the rad(s) and pump with the fans blowing in from the back.
All in all, you'd use about 12U, or a quarter of the typical cabinet, on your cooling components. It would still leave you 36 U for components, but everything would be assembled in a nice, easy to maintain form factor.
My $0.02