It seems the Trek mt220 is a preferred frame to start with.

I wouldn't say being the most common makes it preferred....

The most important considerations are if you want disk or not... and if you want a usable fork with the bike or are just going to buy one ...

Any decent aluminium frame will end up with about the same bike on a 24"
Ultimately if you (might) want disk brakes then a frame with mounts is preferable to an adaptor...

best bang for buck will be to just build one??

If you get everything discount ...
You can certainly spread the cost and use an existing spares bin but when I add up what I spent I would have been as well financially paying up-front for a trailcraft...

Incidentally I got a Cannondale Race 24 (new but discounted old model) because it has the disk mounts and comes with the F1rst Air fork....

The only bits of the original are the frame, fork and headset.
I didn't pay full price for anything yet I must have spent the same as buying a trailcraft.

On the plus ... we built a bike together and he can maintain it (well with help) ....
On the -ve it took a long time to get all the parts discounted/eBay and kids are only going to fit the 24 for a limited time..

I spent forever trying to get wheels ... (due to the disk requirement) and got Stans Crest discounted (20%) with Novatec D811/812 discounted (40%) and Sapin X-Ray at 1/3rd retail... but I should have bought a set POAH was selling a year ago instead of procrastinating.

Meanwhile I got some rubbish disc hub wheels ... that cost me etc. and we ended up on a oval chainring so the 30T N/W wasn't used (though just stuck it on one of my bikes)

 

On the 2008's just make sure you know how to identify if the frame is recalled or not. That's a nice looking bike though. I bought one but ended up selling it because it was so pretty and I was just going to tear it apart. I wanted one that would take disc, but that seemed a bit tough to get a steal on at the time so I settled with rim-only frames. Now what I don't like about my girls' hotrock frames, or at least one of them, it doesn't have enough bottle space in the frame triangle.

 

Both my two girls have had a MT220, they are both relatively tall and started riding it at age 7 and had outgrown the 24" bike by age 9. You might want to double check that a 24" wheel frame is still a good fit for your daughter, she might be ready for a XS 26" wheel frame bike (which is much easier/cheaper to source good parts for).

THe recall MT220 to avoid was a girls specific model with a single boom-tube connected to the headtube, no separate top tube and downtube. My girls did fine riding a standard frame.
Info on my MT220 build at;
http://forums.mtbr.com/families-riding-kids/trek-mt220-24-mod-build-894279.html