Nick
A lot of questions.... I'll see If I can remember them all!
Wholesale Trains has Aristocraft commonwealth trucks on sale. See their website under special sale items.
DCC outside can be done... depending on the climate and track type. DC and air throttle, or DC battery and Aristo's Revo RC system probably better, and a lot less track maintenance. Constant resets from outside dirty track will get old real fast.
Ozark castings are generally 1/22 or 1/24, Aristo/USA are 1/29. Close...but no cigar.
For Valve Gear you are on your own. The models that you could use for scrap parts are generally much more expensive than the Aristo you intend to use. Aristo models work fine, just less expensive than the models you would have to scrap to get modification parts.
Aristo's current production power blocks can handle six heavyweight cars on a reasonably flat layout, if your want to pull them up grades greater than 1 1/2% or more than six cars, I would recommend installing ball bearings (available as a spare part from Aristo) in the trucks. Ball bearing integrated axles do not work as well in this particular application.( they work fine in regular freight cars though)
Kadee 830 couplers will work fine on Aristo's long passenger cars, but you are restricted to 10 foot diameter curved track or greater. Don't bother with Kadee's adapter for Aristo passenger cars, just use a wood shim, and cut out the sides of the coupler box!
The 2-8-0 is not yet in production.....Given the current economy, I expect the delivery date to slip into next year. They can't build new trains if nobody is buying current production. Sales are slow across the hobby business.
Hope this helps
(Aristo heavyweight cars with Kadee 830 couplers, close coupled)
Commonwealth trucks on Aristo Tender (modification to tender floor required for proper height)