I just got a 1980 Hobie 18 this past fall too - I've been fixing it up for the past few months, and should have it ready come spring. I'm by no means an expert, but here's a list of some of the things I've done, some of the things I've read that I should do, and some of the things I have no intention of doing that you might want to do
I found on the deck in front of the port cross bar, and 2 smaller ones on the outside of the port hull, a few feet further forward. I used Git Rot on them (works great - no problems). One note though - DO NOT forget to mix in the hardener. I got distracted while I was mixing one batch, and forgot to add it. What a mess that was to clean up...
On our '80 boats, the 2 crossbars and shroud anchors are held to the deck lip with steel "channels", about 3 inches long. They've since changed these to anchor plates that actually use 4 bolts to anchor to the sides of the hulls. There were some problems with the mid-80's boats (you can read all about it in this forum), but for our older, heavier boats, we really only need to upgrade the shroud anchors from the channels to the anchor plates. It's a pretty simple job. Just make sure you use plenty of silicone to keep water out of the holes you drill.
The original access ports on our boats are the "snap-in" type with the rope handle. People had problems with water entering the hulls through the rope-handle holes, so it's not a bad idea to upgrade these ports to the screw-in type. This is a pretty simple fix too. Beware though - when I bought my kit, I was under the impression that it contained plastic mounting hardware. Instead, it came with aluminum rivets. I actually think the rivets worked quite well, but you may prefer something else. Also, apparently the kit I purchased had been sitting on my Hobie dealer's shelf for quote some time, because half the tube of silicone had hardened inside the tube. Fortunately I had another tube lying around from some previous repairs. Don't skimp on the silicone for the access ports either - I used what I thought was WAY too much, and now I actually wish I'd used a tiny bit more.
There's not really a "problem" with the wells, but it's a common place for small leaks to develop, and the boards can wear away at the gelcoat, so you might want to take a careful look at them. When I bought my boat, it had a few gallons of water in each hull, and I suspect this is the entry point. I replaced and re-sealed the drain plug housings and got rid of the old access ports, but I think I may have a small leak in the bottom seal of the wells too. I'll probably seal them with silicone, then do the soapy water test you just did.
Our '80 boats have a mast-step that is attached with 2 rivets. It's recommended to upgrade to the newer 4-rivet mast-step. I haven't done this one yet, and I'm actuall not sure why it's recommended. Obviously 4 rivets is stronger than 2, but does anyone know the true reason for this upgrade?
It's not a bad idea to replace the standing rigging (shrouds, stays - anything that holds up the mast) anytime you buy a used boat. You just don't know how the previous owner used and maintained the boat. My wonderful wife got me a full set of new standing rigging for Christmas. Saltydogmarine has a great deal if you buy the whole set (the trap wires are basically free, if you do the math).
Our '80 rudders are the "old" (pre-1986) style rudders that have a metal cam instead of the standard hobie plastic cams used on the 14, 16, and the other boats. In '86, they changed the H18 rudders to use plastic cams as well. I've read that the metal cams can be a pain to use. I'm going to stick with the old style rudders for now and see just how much of a pain they are before I spend the money to upgrade them.
I don't know about your boat, but at the top of my mast where the halyard ring rests, there's a floppy triangle that sits over the hook. This was designed to make it easier to un-hook the halyard ring, but I've been told it makes it a royal pain to hook up when you raise the main. I haven't done it yet, but I plan on removing it.
I also have some cosmetic work to do still - the boat needs a good polishing and a few coats of wax, and I still have to fill in the holes from my delam fixes, as well as a few small nicks and scratches. And because I got excited about my new boat, I replaced all the sheets and other running rigging as well.
I do have some pictures, but I don't have them posted anywhere yet. I'll let everyone know if I ever get the online somewhere.
I know there are a few other people fixing up H18's too - anyone else want to status their winter projects?