I've used a beta 12a in a bass guitar cab build. It's a good driver, but I'm not sure I would use that dual cone version, in fact, there is little point in doing so w/ a 3500 hz x-over. If you use the dual cone beta and that tweeter a 5k x-over would be better. The standard beta 12 should be crossed at 2.5-3k at most.
3.5k is the minimum rated x-over freq for that tweeter and as such I would want a 3rd or 4th order at that frequency. Like I said, 5k would be about right with a simpler 2nd order. It really should be designed for the impedance of the woofer and tweeter at that frequency though. Off the shelf x-overs are not the best choice.
I would port it. I think my bass cab is 2.3 cu ft and tuned to the mid 30's somewhere. I just used a simple 4" port from PE. If you use a 12" waveguide you will have plenty of box size.
That kit is a real good option. A little more money though.