Ok, here goes.
Yes, you by all means need a 14-71. A 12-71 is an anomaly and you will play hell trying to sell it (which is why it's so cheap).
Littlefield makes some nice alky blowers, but once again, speed costs.
If you can afford a hi-helix and your class rules allow it, by all means. It takes a LOT LESS power to drive a hi-helix than a standard helix.
Also, if you have the cash to afford it, buy a delta outlet. The deltas are a lot more efficient of a design than the standard large bottom opening.
The SSI blowers are bad ass (we run a B-Rotor), but they, like a Kobelco, use a different front cover bolt pattern. So any blower guards, throttle brackets, idler brackets, etc that you mount to the front must have the SSI/Kobelco bolt pattern, NOT the standard roots (GMC).
Go check out RBS Superchargers...
http://www.rbssuperchargers.com/
JR used to work at RCD and started his own company with his wife Robbin. Really good people with a LOT of knowledge. They buy/sell used blowers as well.
Make sure whomever you buy the blower from sends it to RBS (or someone reputable in the industry) to have it freshened. RBS will check it for damage and let you know the repair cost before you buy it.
You can then go back to the owner and make your final deal from that point. If the blower owner doesn't want to do this, then RUN AWAY FAST.
Good luck and welcome to the world of supercharging. You'll never look back.