The delaminations are indeed easy to fix. I've fixed a bunch on our 1980 18.
Things to look for: This depends on the year. You can tell the year of a Hobie by the serial number (there's a guide on the Hobie website under support).
post-88 (89 and on): You should be all set. This boat will have all necessary reinforcements (shroud anchors etc) and enough glass to support the crossbar.
between 87 and 88: You're all set for the rudder, since it should have the plastic cam and the new style castings. However, the front crossbar area needs to be reinforced, since Hobie decided to reduce the amount of glass in that area after 1984. There should be a shroud anchor kit (used as a crossbar anchor kit) installed on the outside at the very least. If this is not the case, you'll have to do this (read: extra work and money).
between 84 and 87: Same as above for the crossbar. Needs to be reinforced with glass on the inside and the 4-bolt anchors on the outside for the front x-bar. These cats came with the OLD aluminium (I'm British, can't help it) cam and old style rudder castings. These have a) a tendency to break and b) a tendency not to kick up when you hit stuff (leading to a) and also likely breaking your entire transom and rudder blade [even more work]). These boats originally didn't come with anchor plates on the shroud anchors. These are absolutely necessary. So things to look for:
a) front crossbar with reinforcements and anchor plates
b) new style rudders (otherwise a rather expensive fix)
c) Anchor plates for the shrouds
The front crossbar has enough glass that you don't need the fix (a) from above. both b) and c) from the 84-87 still apply though.
Things for ALL H18s
Check the crossbars! Make sure there are no cracks in the aluminium, particularly at the blocks for the aft one and at the mast for the forward one.
Check the mast. Make sure it's straight and true. Check the mast foot. If you want to race it, make sure the mast has a comp tip. If you only sail it recreationally, the performance is allegedly better WITHOUT the comp tip. Obviously don't hit any powerlines if you don't have a comp tip. I also wouldn't recommend hitting powerlines if you DO have one, on the other hand...
Check the shroud anchor pins. There is an old style (welded on) and a new style (machined). If they are the weld-on types, you'll have to replace them and they're 54 bucks for the pair.
Double check all standing rigging is in VERY good condition. A breaking shroud isn't fun (and can also cost money, my co-pay for a dislocated shoulder for the emergency room and the physiotherapy was about 300 bucks).
Check the trampoline is in good condition. A new one is 300+ bucks.
Check the sails are in good condition. Sails are about ~150 for a good used jib and ~300 for a good used main. 400/900 new.
Does it come with a trailer? Beach Wheels? Cat Box?
There's more, but I think I've already written too much here..