I think the current 24" kids bikes offer a pretty promising base for conversion. I set mine up with 650c wheels and it seems like a real good seating position and is fast. My only concern is for those with long legs. I think you have to run the telescoping top tube (TFT) out too far. But, it's marginal for me and I do have long legs. Here's a picture of that bike:

I also had mine set up on a cheap Pacific mountain bike frame and while a little heavy, it was a nice rider.

The seat was a little bit higher than I really wanted and I could never get the recline and the nose of the seat just right. Otherwise, it was a hoot to ride and the mountain bike suspension really made for a nice ride.
Mostly look for your frame. Chances are, the components will be throw-away. I guess it doesn't hurt to build it with what comes on it, you can easily upgrade later. I bought the mtb frame from a Craigslist dealer that bought bikes from police auction. It was incomplete, had a 24" fork/wheel on the front and had obviously been beaten badly in it's previous life. I already had parts for the build, so I stripped all the junk parts off of it and put them directly in the dumpster.
Mark
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Bicycling is my drug of choice. Followed closely by beer.