I haven't used goof-off, but when I use Easy Lift Off, I keep a big bowl of warm water right next to me so I can rinse the figure every minute or two.  If I don't, then it yellows and eventually cracks the plastic.

I can't tell you what's the BEST to use, you have to decide that for yourself.  Different types of Sculpey have different properties: some are more firm, some softer, some are tacky, some are smooth.  Some people prefer one property over another.

I've never used Ultra Light, or Original, but here are my thoughts on the others:

Sculpey III - Too soft for my tastes, so it was real easy to accidentally mess up an hour of work with the slip of a tool or finger.  That said, it was a good medium to begin sculpting with, and it's cheap.

Super Sculpey - It's a tad firmer than Sculpey III, but one drawback I didn't like was it is slightly translucent.  Seeing the part underneath what I was sculpting made it more difficult to find flaws in my sculpt.  Ultimately it was too soft and not as tacky as I like clay to be.

One cool thing is you can boil both of the above in water to harden it, which is helpful when you sculpt onto a plastic piece that can't be baked in an oven. 

I started out with just plain old Sculpy, which comes in all the funky colors.  From there I played with Super Sculpy, which is a sort of pink fleshy tone, and will set you back 10 or 12 bucks.

There is a sculpting guild here, but you have to be at least a level 10 Mage, have at least 200 gold, and be able to cast fairy fire causing +100 damage.  You must also have completed Hrothgar's Nibelung Quest.  Once all that's been done, you'll receive the secret handshake instructions via pm.

For you to tell me how what a great job I did.   

Great work putting it together, Brent.  The paintjob is very well-done too - it looks like it's seen a sandstorm or two.

Damn, I'm sure the wife's enthusiastic support of your hobby just cooled a degree or two.  At least it can be pulled off easily enough once it cures, right?

Knowing that hindsight is 20/20, and not trying to sound like an A-hole, for something that big you might want to use a wooden mold box secured with screws.

The guys at Goregoregore.com have a good mold box tutorial with pictures.  It's a little overkill for molding smaller pieces, but for something large, it's perfect.  Goregoregore.com's tutorials section is large, but check out the "Molds" section.  These guys do a lot of professional sculpting for Sideshow and other companies, and really know their stuff.

Spirit Iron Knife at Yojoe.com

Spirit Iron Knife at Hasbrotoyshop.com -- same head, different body than the one listed above.