Where are you locatedIt is important to decide how you would like to divide your budget. The smart money is spent on finding something used, where someone else has already taken the hit on depreciation, using that truck and deciding exactly what you want and don't want.
I think that you are going to struggle to find or build a capable FG on your budget. If you have the ability to buy a decent used truck and do all of the build out yourself, you have a better chance, but it will still be tough.
I see so many people who want "everything on a budget" and they end up with cheap/inferior parts that end up failing in some way, ruining a trip or an experience with that type of product.
If you could find an FG with an insulated box in good shape and build your own interior, you have a good base truck to modify as you see fit. If you can find a clean FG with a standard box, consider swapping that box for an insulated box, as insulating a standard cargo box can take quite a bit of time and effort to do properly. Time is money.
I can build you a shell and ship it to WA, but that may not be cost effective for you. A box 6.5' wide x 6.5' tall and 12' long weighs about 400 pounds, empty. This four season box is very well insulated and can be built quite inexpensively, under $3000 in foam and composite skins. This presumes that you have a work space, some tools, time and are keen to DIY the box/truck build yourself.
This is a simple, basic shell, no mounting plates, no doors or windows, no exterior cage, etc. It's just an empty box that requires complete fit out.