I'm actually building a 12vDC system myself, so I've done a bit of research here. I specifically chose the i7 T-series (35w) CPU, B360 chipset, & GTX 1650 combo especially for peak efficiency. If you are one that needs to under/over-clock, then the setup I have chosen won't satisfy you, but the options are a bit different depending on your power needs.
I've had to learn a lot about this on my own, so I maybe wrong, but you should look at the PSU as two pieces. The electronics want a nice tidy 12vDC, and the battery and charging system is going to be anywhere from ~10-14vDC, so the first thing to look at is cleaning up the voltage.
Mini-Box DCDC-USB-200 is only 200w, and runs @~$60
EDIT: This is an all-in-one solution that will regulate and supply power to the MoBo without the need for an additional regulator or Pico-PSU.
Victron Energy Orion-TR 12/12-18A is still only 220w and goes for ~$116
Once you've cleaned up the power to a nice flat 12vDC, then any Pico-PSU that functions on 12vDC will work. I've got a G-unique, and they seem to have a pretty good rep.
Sadly neither of these can deliver the ~250w that you'll need, but your GPU is a bit power-hungry for a mobile unit. If you were to switch to a T-series i5-8600, I believe you would lose the ability to adjust clock speed - thus the more efficient B360 board becomes relevant, and you could probably get away with the 220w setup. Otherwise, I know it's a bit round-about, but you could get a 20vDC regulator (something LIKE THIS), and run an HDplex 400, but you'll be losing efficiency again in the conversion. I wish I could be more helpful, but I'm kinda' learning as I go, and this is my first 12vDC build (thread HERE). Hopefully someone with more knowledge on this can chime in, and we can both learn something. Cheers, Cotton
