I took a look at the specifications for this Fuji bike as I am sometimes asked for recommendations for a kid's bike. I not particularly impressed and probably wouldn't recommend it. It has only 7 speeds. The gear range for a 650b X36mm tire, a single 40 tooth chain ring and 14-34 cassette is 30.3 to 78.6 gear inches. That assumes a 170 mm crank length but it probably is shorter on a kid's bike. That means an even lower gear range. Even kid's mass merchandiser junk bikes rarely have only 7 speeds. No mention of the weight in any review and not on Bikepedia where I couldn't find it listed for 2013 or 2014. The Tourney RD may be OK but it is either bottom-of-the-barrel or pretty close. You can buy a brand new one for 13$ on ebay, shipping included. You can get the bike for under $400 but it seems like it has too many compromises made to keep the price that low.

You seem to be knowledgeable enough about bikes to follow up on the build-it-yourself idea. Seems like a better way to go.