I think you may have a real jewel there. The boat, trailer and the fact you have wings is a great deal. Personally, I do not like the wings but that is mostly because I never had them. They are great for day sailing and folks that have them, would not sail a boat with out them.
So, others have cautioned you about the areas of weakness to be concerned about.
My take is you have some real potential for great sailing at a very low cost. The bang for buck versus other boat situations is the best you will find with what you have.
As far as parts you need to get the boat back into shape can be found on Ebay. I locate all my Hobie 18 parts there. Many of the dealers advertise their parts there. Sometimes used stuff, sometimes new stuff. Be patient and spend some time looking. Your most expensive items will be dagger boards. However those can be very inexpensive if you find the right deal. Do not worry about a little damage on the boards they can be easily repaired with epoxies these days.
First clean up the boat, various beach cleaners. Next item is a new tramp. The one you have can be used with out issue, but good news is they are not that expensive either.
Your jib can be repaired. I just had a new zipper put on one of mine by the local sail shop.
Now a word about "soft hulls". I think this topic has caused way more "concern" than is necessary. Here is my "experience" with old Hobie 18's and "soft Hulls". I currently have 4- Hobie 18s. I have one for each member of the family. At our local Club races if one of my family members can not make it out on the water, I get who ever wants to take one out to come out and sail a boat. So usually all 4 are out sailing. Now all of these boats are aged between 1979 and 1982. Some decks are softer than others. The 1979 is the oldest, heaviest and softest of the boats. My plan was to just sail it until the hull either cracks significantly or breaks, mainly to determine how big an issue this soft hull concern is? The reality is the boat just will not die. Now I have been sailing it the most. I have been sailing in the Ocean too off of Daytona Beach, in and out of the surf. It has been turned over many times by my daughters and it just keeps on ticking. And yes, the hull decks are still soft. So to shorten up the story a little, go sailing. If the hulls seems so soft that it is scary, then put in a port hole and add some more glass. I am of the camp, that I doubt it will need that.
The wires that hold up the mast should be replaced if they look rusted. We just had a mast fall down this Saturday when we had all 4 boats out sailing. The reason was the pin in the side shroud came out. No biggie, put the mast back up and we were out again. So probably more important to tape the ring dings so they don't come off. On that boat, when we towed it back to shore by powerboat by the front bridle, the front bridle had spit half way at the attachment. So now it is time I should replace some shrouds on that boat. I still sailed that boat the rest of the day with wind gusting to 21. Point is, these boats can take a lot of abuse and still work well.
Just to back up my confidence. If that boat is of too much concern for you, I will buy it from you for what you paid
Good Luck and many happy years sailing. By the way, I have been abusing that 1979 for about 4 years now and still no issues.