PROGRESS UPDATE:
I finally found the right bike. Picked this up for $100. Needs a good tune up but the size is right (rider is 5"3") and it has some good components. Marisocchi fork, hayes hydrolic disc brakes. I dont know what the frame is, the only marking says its 6061 heat treated tubing. Needs a good tuneup, new brake pads, shift cable but I am really happy with this bike for the build.
I also got started on building the wheel. I ordered the Sapim strong spokes from Grin ($46 plus shipping for 36 spokes). I found it quite hard to find a 36 hole rim to buy - called every local bike shop with no luck. I opted to use a good used Alex dm18 rim that I already had, and it was a preset option in the spoke calculator so I didnt have to do any measuring. Unfortunately Grin shorted me a spoke, only sending 35 spokes. I got the rim laced up but cant finish the wheel until I get the final spoke. I contacted GRIN and they already sent out two spokes (so now I have a spare at least)
I also picked out and ordered a downtube battery box and controller. I had some thoughts about doing a custom shape diy battery box, but ultimately decided to go with a premade box in order to make my first battery build a little eaiser and keep it sturdy and safe. I found this Polly DP-6C battery box that can have a kt controller built in to the battery base. I really like the idea to keep the build very clean and minimal, and to make it easy to swap to a different bike if I want to.
I ordered through the topbikekit.com and was able to get everything I need in one place. Came with the battery box, cell holders, precut nickel strip for a 13s5p pack, 30A 13s bms, KT 22A controller, lcd5 display, half twist throttle (slim version that wont interfere with trigger shifter) 9pin motor extension cable, 2to1 cable for throttle and display connection. I am going with no pedal assist or brake sensors, just throttle and display for simplicity.
I measured out the battery box and it will be a tight fit. Here is hoping the actual box fits. The drawing only gives the overall height and length, so those dimensions are accurate but the tapered angles are just eyeballed from the drawing. And I know it will need to go further up the downtube in order to slide down into the holder, so... I may have to get creative with mounting. There was a smaller 52 cell hailong box available, which would be fine except that the only compatible controller that the seller had was only 7A continuouos 15A max. I really wanted to get the 22A controller on this build. So all I can say is I hope it all fits!
I'll update in a few weeks with everyting arrives!