If you only put seven or eight drives in the backplane your one PATA cable would be fine, but I imagine that in looking at an HL15 you are also thinking about adding more drives. If you plan to add AI GPU(s) to the mix, I’d spend the couple hundred extra for the chassis with PSU unless the VAT and import duties are particularly onerous.
There’s a development picture of the XL version here; **Sneak Peek Alert! – HL15 Beast Incoming **. The intent is to support the largest GPUs and EATX motherboards, so it is going to have to be 5U and probably about 5 inches deeper (25 inches total). It isn’t going to be compact and space efficient. Right now, I don’t think they’re looking at a version of the HL15 with its current dimensions, but trading some of the drive bays for longer PCIe cards, eg.
In the HL15 v2 full build you could run dual 16 GB GPUs to get 32 GB VRAM. There are 16GB GPUs around, like some versions of the 4070 Ti Super or AMD Radeon Pro’s that will fit in the HL15’s motherboard chamlber.