Any high quality modern PSU is a rail to rail design (DC to DC).
All of the AC voltage is converted to 12VDC. Then if needed some of the 12VDC output is converted to 3.3V and/or 5V.
Like p_Shep said it is a thing of the past. The one exception might be some "non 80 Plus" generic POS power supply. Anything from a solid brand should be fine.
As far as loading only "one" 12V rail most (all?) DC to DC PSU only have 1 REAL rail. They then use current limiters to split that into a bunch of virtual rails (so someone doesn't try and pull 102A+ through a single 12V wire).
Loading only 1 "rail" shouldn't be a problem.
The main thing is to ensure good efficiency (and poor efficiency is hard on the PSU switching circuity) you want the load to be at least 20% (and 35% to 50% would be better) of peak. So don't go buying a 1200W PSU to power 2 FPGAs.
Lastly while 80-Plus Gold may not be a huge need for FPGA (given their already high electrical efficiency) the higher end units tend to be built better with "beefy" Japanese solid state caps, heavy busbars for power distribution, and higher end fans. Then tend to have 5-7 year warranties so the manufacturer is really designing them to perform well for very long time (or their profits get eaten up by RMA costs).
IF it were me to power $2000+ in FPGA I am looking at a short list with SeaSonic at the top.