> Many rack space rentals will not permit you to just install whatever PC you fancy because it is potentially a risk to the neighbours in terms of fire or bad hardware, most will happily quote you to buy one their approved ones!

I have _never_ experienced this. The only restrictions I've seen on colo contracts I've gone after were related to UPSes and things with large batteries in them. So a big stack of laptops would be a no, but if I wanted to put Atari ST's or Dell PowerEdges or white box builds or bitcoin miners it doesn't matter. I guess I've always done things at at least a half or full cab, never single Us at a time.