Plus 4 would be the IDEAL novarossi in my opinion for a buggy. The p5 is a great motor, I would say more geared toward a truggy then buggy. I have however seen multiple people run it in buggy with great success.
I have not tried the new Vspec but you could alway settle for the old one which is now avaible from a few places for $200 which at that price is a steal. You won't find a better motor at that price. It has just as much if not more power then a plus 4 but not close to as smooth or as good on gas mileage as the plus 4 when I used it.
I have not tried a B6, do not plan on it, haven't seen one that has made me want to get one yet. Not saying it is a bad motor, just haven't tried it.
RB Shark - Have not tried it, nothing to say about that motor.
The new vspec I am sure will be great, but again have not tried it. I do plan on getting some new OS Speeds for next season, we'll see how that goes.