Before I started on my 48 truck I sat down and estimated what everything would cost, then multiply by 2 or more to account for all the little things.
I knew before I started that I could spend less money on a new crew cab F350 Diesel than I will have in my old Chevy truck when done. The benefit of building your own is that you can use the best parts and design things to suit your needs and wants. When done you should have a more durable vehicle, and something you can work on and understand from one end to the other and from inside out.
I am building it to haul anything I would want to hook behind it, not to much different than what you want.
Grigg