I currently play a Hamer B12S. It cost a lot, but I think you can get them for under $2,000 if you don't run up the price with a lot of options. (I got mine in 2004 so I'm not current on pricing.) Of course, the production regular scale Hamer 12s come a lot cheaper.
You won't be able to play a 12 quite as fast as a 4. You won't probably be using it on all your songs. I've had two 8s previously, and I think the 12 is a better balanced sound. The two octave strings balance the volume of the bass string better without you having to dial in a lot of clanky highs, and you get the added benefit of phase cancellation between the two octave strings giving you the "12 string sound". I recommend you tune the bass string dead on, one octave string a couple of cents low, and the other octave string a little high, but by ear, not the tuner, to get the most pleasing sound.
I like a short scale twelve for these reasons. The bass notes are rounder on a short scale generally. The octave strings will provide plenty of clarity, so you get a fuller sound.
The strings are floppier. This is a good thing when you are pressing 3 strings at once. Also, when you dig in, you'll get an almost "slappy" tone from the multiple strings.
The faster "get around" on a short scale helps make up for the slight slowing of pressing multiple strings.
I previously played my eight strings with 3 finger picks and a thumb pick. I play the Hamer 12 with 3 bare fingers and a thumb pick. The string spacing is narrow on the B12S and I like this because it lets me play chords better.
Hope this answers some of your questions.
Shameless plug: I'm thinking of putting my Dean Rhapsody 12 in as-new condition up for sale.