A regular Hobie-18 with wings is a Hobie-18 Magnum, unless it is sporting the taller mast and rig, then it is a Hobie-18 SX.
The NACRA 5.8 is a great boat. The NA refers to North American. The NA rig features an aluminum bow foil that attaches to the bow bridle tangs via short rigging - the forestay attaches to the middle of the foil. It's a way of getting a lower tack point for the jib, and therefore a bigger jib. Regular 5.8 has regular bridle wires coming together to meet the forestay, jib tack is a little higher, jib is a little smaller. NA gear can be retrofit to a non-NA boat. So, NA = bow foil and bigger jib. Assuming the boat isn't parked out in the field fully rigged, you'll have to see the gear that goes with it.
The older boats do not have foam core in the hulls. I forget which year the change was made. The newer foam cored boats are worth more, but a non-foam boat is plenty good. Just look in the inspection ports into the hulls, you should be able to tell. Assuming it's in good shape, I'd still get it if it's not foam, but you might use that to get a better price.
The 5.8 is faster than the 18. It has more freeboard and handles higher crew weights better (especially in rougher water), and the hulls lack the drag inducing deck joint that the 18 has. The 5.8 will weigh less than the 18, although the 18s range a bit in their weight. My 18 felt far heavier than the 5.8s in our Fleet. But then it felt heavier than my van at times...
Speed-wise, it's faster than the 18, similar to the Prindle-19, a little slower than the Hobie-20, a bit more slower than the Nacra 6.0na. Similar in rating to a NACRA I-17R, Taipan 4.9 as F16.
It's been a successful boat in our Portsmouth Fleet, at times we've had three or four, one guy took top honors a couple years in a row on an older one. We've had one that the skipper regularly single-handed, always a threat when he sailed.
I'd say if it's in good shape, buy it. Especially if the local Fleet you want to sail in is Portsmouth. But if you're looking for one-design action you probably won't have too many others to play with these days.
