You may also want to post up on mtbr.com under the tandems section. There is a wealth of knowledge over there regarding mtb tandems.

Regarding your build, how much does your team weigh? Has a big impact on the wheel and fork selection. One bit of advice I'll give is to not skimp on the rear hub. If you intend to do a lot of climbing, it is likely that you will destroy your rear hub at some point. Chris King has been the only one that has not left us walking our bike back to the car (or the downhill towards the car).

Yes, Middleburn and DaVinci are pretty much your only options for new tandem cranks (if you want to be able to run a proper granny gear).
I've had good luck with Maguras. We are currently running MT4s. Also had good luck with the older Louise.

We're on our 3rd mtb tandem, so I get a little geeky when it comes to the off road tandem discussion!

Feel free to PM me, or look me up over on mtbr... I'm ds2199 over there.