They handle rougher water than other 21 foot and smaller tunnels, but not the stuff the 24 would handle. Some people say that the 21 is prone to barrel rolling. I think they are exagerrating and talking fifth hand experience. The 24 and the 21 are not built for sharp turns at any speeds. Basically, common sense an seat time will keep you safe.
With the 200 I would guess you should be in the low 80s, high 70's maybe. They were built originally for the bigger horse V6's and V8's of the time. I would look for a 300x to replace the 200 with.
Skater built 4 21's with twins. They really ran. There's a guy on this board that just sold his 21 with twins, (LambRB), it ran 120 with twin 2.5/260 offshore's and 133 with twin S3000 powerheads.
Run cleavers only on this boat, they don't need any bow lift out of the prop, the hull takes care of that.
As far as what to look for on the boat. The usual; stress cracks, rot in the transom, but I doubt you'll find any. Skater's are in a class of their own.
You won't be disappointed.
They are not for everyone, not alot of storage, not for skiing, not for the faint at heart.
Powerboat Magazine has the 21 Skater in these issues;
3/91, 5/91, 4/96
You can order these back issues by going to www.powerboatmag.com and looking under "e-commerce" for ordering instructions. It's like $10.00 an issue after tax, S7H, etc.