#1
I've finally gotten tired of hauling my Randall 4X12 to practice or using a combo that just can't compare to my RH 200SC, so I've decided to build a 2X12. I'll be building as close to Vader specs as possible.
18"x 30"x 14"
3/4" Plywood
wired to 8 ohms
Not sure of the speakers, maybe Eminence Legends or Celestion Vintage 30s.
You think I should go through with it? Any which speakers to you all think i should choose?
Bassist/Vocalist-Visceral Violation
Mesa-Boogie Big Block 750/SWR Megoliath
Warwick Corvette
#2
I built a 2x12 a while ago with a set of greenbacks and it sounds great. It was a bit stupid though because it was actually less portable than my half stack due to it not having castors or handles. Both the speakers you suggested sound great and if you want something more portable i say go for it. 2x12 is way better when trying to fit in a car and lug around.
#3
Go for it. You need to match speakers to cabs so either choose the same units as in the cab you like or be prepared to ask what size cab after choosing the drive units. Open backs sizing is much less critical but only some speakers are suitable.

I've written a couple of articles on UG you might want to read. This is the easy one http://www.ultimate-guitar.com/columns/gear_maintenance/cabinets_for_guitars.html

Have fun