If you really want the smoothskin there is another thing you can try. I had this old ugly 1pce nylon stinger suit that I would wear over the top of a smoothskin and used it for scuba. That way you are much less likely to rip or tear it on rocks or taking the scuba gear on&off. Would have to be a slightly larger as a well fitting nylon over a 3mm may still be slightly too tight. Some other alternatives not as good but cheap - tshirt and shorts over the wetsuit.
I do like the smoothskin but even for normal spearfishing you are probably better of with an outer lining. Sure for a depth competition the extra few % saved in drag is worth it but when spearing, not such a big difference....
Other factor you have to consider, I have scuba dived quite a bit with freedive suits. In general at depth the freedive suits become less warm then a scuba suit because they compress far more. ie on surface much warmer, seals better, but at say 20m even a 3mm freedive suit is about 1mm or less. The scuba suits aren't as stretchy but most have much better compression resistance. Probably the only freedive suit which I've tried which is pretty warm at depth is a Elios - Heiwa high density. It's less stretchy then the low density, but far warmer at 10-20m. Mine is a 3mm with superstretch inner lining. When I say less stretchy then the low density, compared to most spearfishing suits it's still much stretchier. I have used it for competition training and done depth dives to 80m so it's not such a huge performance difference vs a low density suit as some freedivers will tell you. What does make a big difference is with a low density suit because of the compression you start gliding much earlier, which some divers definitely like. But this is what makes it colder....
Whatever you get, I definitely recomend a custom made suit, once you get one you won't ever wan't to dive in a stock suit again.
Cheers,
Wal