Yeah, was best man at a wedding yesterday, so I wasn't exactly on top form for the last two days.... I do apologise.
OK, working backwards...
The SC846 would be great, I think the 2u is the SC826, I'm struggling a bit on prices. I'm not sure if I'd need a SAS controller to use it or if the backplane gives sata access.
Which leads neatly to the H310 HBA, I could only find these on a 2nd hand site 'new' for around £30, seems pretty reasonable, 4 port sata cards seem to be a similar price.
I'm still not familiar with SAS, having never used such devices, should the need arise, can I simply use SFF-8087 to 4xSata adaptor cables with the H310? (I have assumed yes further down the post...)
Up next is the "2630L/x99 board/8GB/Antec 900/H310 HBA, 3 x 4in3 backplanes" build.
The motherboard/CPU/RAM I already have available, so this works out cheap so far. The antec 900 seems to be about the same price as the silverstone case which has a backplane already in it. I did look for the 4 in 3 backplanes, I was considering adding those to the current PCs and spreading the drives around as a cheap option, but the best I could find was about £50 each, 2 of those and I figured the silverstone would be easier, though potentially more cabling as the connections aren't sas, but at least I'd only have to cable it once and forget about it.
So as it stands, I think the silverstone case with a H310 HBA is the way forward since the only extra power consumption is the H310 and any new drives I add. With 8087-4xSata cables the parts cost is about £140 all new parts (excluding the bits I already have, obviously).