If you want to get the SC-1mk2 model, you can fit 4 channels in a 1u rack. But you don't have any VU meters.
If you want to get the X-12 model, you can fit 2 channels in a 1u rack, and you get VU meters.
Both the SC-1mk2 and X-12 have different sound, the X-12 is a bit more expensive too since it has both input and output transformers, more relays, more switches, bigger board, etc.
PSU? VU? The kit? does it have to be ordered separately or does every preamp kit come with everything is needs?
All the kit comes with everything you need. The X12 kit for example includes all the knobs, and VU meters and XLR jacks. The SC1mk2 kit includes all the knobs and XLR jacks (no VU meters).
The PSU and Power Transformer kit includes all the fuse, power switch, transformer mounting kit, etc.
Let me know which model you want to build and I'll be able to tell you what product combinations to order.
I also have packaged deals... for bigger savings. For example, order (2) of the X12 and get the PSU and Power Transformer Kit for FREE.
The SC-1 is clean and clear preamp. The X-12 is more inspired by an API design with both input and output trafos. The SC-1 with an input transformer is still clean and clear but some beefiness added to the low end and silkier highs.