Looks like a good set up chap, load your PC up with all the stuff you'd take out into a game before you adjust it so it'll sit right and be comfortable.
The belt and harness combo I use is Viper, adjusts out to a 52 waist and down to 30/32, it's comfortable and the harness offers lots of adjustability, would sit under your PC well. You could also use a pair of normal suspenders if you wanted something slimmer/smoother under your PC.
I have 2 different belts and swap the harness over depending on if I want to run my VSR/L96 or MP5. One is laser cut molle and the other is molle loops and I can't say I notice any difference in them aside from how they look TBH.
I used mesh cause I'm a sweaty bastard and fog up goggles within minutes of being on the field, I don't think any normal anti fog unit could keep up unless it was running big fans. A helmet will only make the fogging worse for you but will give you a more comfortable place to mount an exfog unit.
I've never liked external knee pads as I could never get them to sit comfortably and be on the right position so didn't bother with them till I got trousers with inserts, they will help with the baggyness you suffer if you can deal with external ones though.
You could always run a dropleg holster if a belt platform wouldn't work for you.