ISO numbers sre your best friend for tire sizes. There wre multiple 20 inch tires that are all different sizes, which makes no sent to me, but the local bike shop told me use the ISO number and it will all match. I believe my 20 in rims, and the most common now days are ISO 406. I looked for an ISO 559 rim, because i already have the tubes but couldn't find them easily so i went with what would be common and easy to find and replace.
I had the same issue with headtubes. When I bought the bearings and headset kit it was around 40 bucks each and only some aprts matched the bikes I had for parts. I learned then unless you already have them, don't mess with looking for matching donor bikes. Go to target or walmart and get 2, 100 dollar bikes and be done. You'll have matching head tubes, and the correct parts for each, 2 chains (you'll need 3, which at 16 bucks each, thats 48 dollars for chains if you need to buy all 3) crank guard and set of pedals and extras of each. The shift and brake cables are new and can be used, the pdf says to buy those new for safety anyway, each run about 10 bucks. The frame builders sell the heat tubes for roughly ten bucks. Front and rear derailleurs. Add it up, 15 chain, 40 head set, 10 brake cables, 10 shifter cables, 10 head tube. You're at 85 already. Now what about handle grips, bottle holder, resue the goose necks, pedals, crankset parts, a v brake insead of a rear disc brake? those aren't all mandatory or in the plans, but were mods or parts I made when I started riding that I bought costed money that would have came with new donor bikes. It's just my 2 cents but I wasted a lot of time looking for donor bikes that matched then tried to find parts to fix the bad pieces of them, cleaning and oiling, only to go for my first ride to find out it was the weak link and i wanted to replace it with a new part anyway. It's somewhat disheartening to realize all your work and effort, and you have a new bike with some old parts that don't quite work right.
Also you'll need a 3rd head setup for the steering. It doesn't need to match. I bought that one used, i looked online for kids bikes. The huffy kids bikes use the same headtube parts and size as adults and those are always being out grown and easy to find with very little wear on them. i Found two and they were 10 bucks each. If I didnt i was going to just get a new kids bike at the store also.
I had my wheels laced up at the bike shop and bought the rims and hubs online and i can post links if you want. Others have said its easy to lace up yourself, for me i wanted to keep building and delegate that off so it was still "rolling" while i was building. It would have slowed me down to learn to lace them, get it done, then get back to building etc. But your mileage may vary.
I have a thread on here about the things I learned along the way, shortcuts, etc. that will make it quicker, easier etc. I would read though it, Jons build thread and use the way back machine and look at spinners also. There are links in the afore mentioned threads.
I am just finishing two more warriors and will be updating my main build thread soon. My first warrior took from Nov 24th to March 1st for the first test ride (had to learn to weld and holidays, vacation, some other delays along the way) These two I started on July 17th, the first one went for a test ride sep 15th, the second will be in the next 2 or 3 days.
There is a saying about building something, you're 80% done with 80% to go. I felt like I was never done building the first one, but was always almost finished, just one more thing.... To help with that feeling, the next two trikes, I tried to make them like Ikea furniture. I cut, welded, built, drilled, tapped everything I could ahead of time so that I had a box of parts ready to go, so when I was ready to build the frame or axles, etc. the individual parts I needed were done. An example, the brake supports were cut, capped, welded, ground smooth, the steering tabs were cut, rounded off, drilled and welded on the brake supports. So when I got to page 126, I already had 126-131 finished, I just had to make the fishmouth cut and weld it on.
Anyway, that's my "nickles worth of free advice", now off to finish the 2nd warrior.