$2k will get you a decent bike, if you don’t include the donor or can get that cheap. I’d first decide mid drive or direct, since that would impact how you allocate funds.
I’m spoiled by not having to shift, so I like direct drive, so this is how I’d allocate if it were my money:
~$1k on a battery 52v or higher that can discharge 80 amps+ continuous.
$500 on a Phaserunner & CA v3.1 display combo and accessories (switches, etc.)
$75 on good torque arms
The rest on a good efficient 1500W+ motor (maybe Leaf or WCEC).