If you’re not going to use regular longboard trucks, you need tipped ends or angled risers. Channel trucks like trampa/mbs/kaly/the ones in the for sale thread you linked, need around 35 degrees of angle in order to turn correctly, so they won’t work on the deck you have w/o angled risers.
Anyways, your first hurdle is going to be coming up w/ an enclosure that fits the deck.
An AT build for $800 is pretty much impossible. I’ve got what I would consider a bare minimum AT build, and it rings up at about $1.3k paying off the shelf prices. If you’re as patient and lucky as I’ve been, you might be able to get it closer to 1k.