I have been (verly slowly) building an eBike for myself. My goal is to keep the cost as low as possible while having a bike that I can take to the store and back at least. We live in the foothills, so casual bike rides are possible but take ironman-like strength.
I started with a bike I found at a thrift store for $30.00. It is a Marin bike from the 90's: quality but old and needed quite a bit of loving.
After finding a front-hub on ebay, I ventured into wheel-building and managed to assemble the front wheel with the hub (lacing they call it in the bike world).
Next up is purchasing a controller and display from China.