there are a few things that need to be considered. power consumption, heat, space but you already said that is not an issue.
The more powerfull the PC the more power it will consume. A 90 or 150 watt 12vdc power supply might not cut it. you would have to find something bigger or go woth an inverter. The same with heat. more power more heat. it will need to be mounter where there is an ample amount of air flow.
I have a full blown PC in my truck. I also have a yukon with tons of room to spare. I use an inverter for power and there is plenty of air room. I think most of the people that use the SFF boards want enough power to do what they want it to do. I on ther other hand need AGP graphics, pci sound card and a few other things not found on most SFF boards so a full PC was my only option.