Hey Nuts, looks like I'm on a similar path as you. I was kind of thinking the fat bike thing was a bit of a fad/bad joke but my curiosity got the better of me. Well I am glad I did it now. These things are a lot of fun.
If I was limited to only do one change on my fatty it would definitely be tires. I know that the best fat tires are like double the price of good DH tires but they are so much better than the entry level tires that come standard on a cheap bike.
I put a pair of Schwalbe Jumbo Jim 4.8s on my bike and the difference they made to the ride was amazing. Worth every cent IMHO.
I'm a bit worried that your dd hub might get a bit hot during long slow climbs, this was why I went for a Bafang 750w geared hub. I ordered the cassette version so I could throw my 10spd 11-42 sunrace cluster straight on. They also come with 190mm dropout spacing on the axle so it should fit straight in.
Oops sorry I missed that you are doing 2wd which should keep the heat in check.
My costs are a lot higher than yours so far all in AUD,
$699 Wheeler Savana fat bike
$50 Sunrace 10spd 11-42 cassette
$195 Luna Lander fork + $77 shipping
$240 Pair of Schwalbe Jumbo Jim 4.8 tubeless ready
$50 Venzo DH stem and h/bar combo
$645 Bafang 750w 190mm cassette geared motor kit from Conhismotor
So just short of $2k and still no battery, I will just use my old faithful 12s 15a/h hobby king lipo pack to start with.
Any ways sorry to hijack your thread just thought I'd share the fat love
Good luck with your build.