Utility Cycling - xtra w/ electric assist drivetrain suggestions?

I have an xtra-cycle, and I'd like to add an electric assist (stokemonkey) to it. I'm also inclined to upgrade the drivetrain from the 7-speed derailleur system, but mostly because I want to have a disc wheel in back rather than the v-brakes. I have a feeling that a rear disc would be better than rear v-brakes if I'm going to be adding some juice.

So my options, including pros/cons:
Leave drivetrain as is.
Pro: least expense, it works, has decent gear range
Cons: not sure about braking with extra speed/load. I've had no issues to date, but????

Get a 8/9 speed disc hub, a spacer, and keep the 7-speed.
Pro: Next least expensive, gets me disc brakes
Cons: I've had trouble with these spacers before, I'm concerned about not getting tension right with add'l torque load.

Upgrade to a 9 speed derailleur system
Pro: no more fiddling with spacers
Con: have to buy new shifter as well (derailleur is modern XT, so wouldn't need that), any thoughts on the durability of a 9-spd chain with the extra torque load from e-assist?

Go IGH (Alphine 8, SRAM 9, Nuvinci, Rohloff is too $$$)
Note: there's a whole discussion on this already, so rather than splitting out WHICH IGH, just consider any of them, and defer the pro/cons of particular IGHs to the other threads.
Pro: chainline, simplicity, durability
Con: Cost, I have less experience dealing with IGH systems

Right now I'm kind of leaning towards the IGH route with either the Nuvinci or Alphine hub, but I have no idea why. I'm thinking my best bet is to go with the 9 speed upgrade, and maybe even get buy on the friction shift setting on the 7-spd xt thumb shifters. I bet it would cover most of the gear range. Anyway, I'd like input from those of you who have more experience with these things. First, should I even worry about disc brakes vs v-brakes in the rear? I guess if the answer to that is no, I have no reason to consider the others questions.

Any advice out there on this?

http://homepage.mac.com/awcg/.Pictures/XtraLite/XL1.jpg
I'm running a NuVinci.
I've cracked many hubs. The NuVinci is the only one I would use on an electric/xtra setup now.
I beat the **** out of it and it is the only one that has held up (I ran a Rohloff for a short time on that bike but took it off when the NuVinci came out, did not want to risk cracking the Rohloff, too much $).

That bike also has V brakes. They suffice. A disk up front would be nice but not necessary on the rear. One thing to take into consideration is with the weight and long tail it can be easier to lock the rear wheel. The NuVinci does come in a disk version, though I think you could save the cash and use a rim brake.

+1 on the NuVinci advice, but go ahead and get the disc version - it's not any more money for the hub, and you don't have to mount the disc if you don't want to. That way, you keep your options open. Check here: http://tinyurl.com/938zdt for Bike Hugger's first impressions of riding a Stoked NuVinci.

My roommate uses a crystalyte 408 hub motor with a 35 amp controller, and a 48v10.5amp lithium polymer battery. it has helped him out a lot since he was hit by a car. it can really scoot uphill pretty quickly, even with a full load on his xtracycle