After owning more 18's than I care to admit, I have seen soft, rock hard and fraggle boats. The toughest boats I have found are the early 80's white boats. Heavy for sure but built like a brick s@%# house. The older colored boats all seemed to absorb the heat and can go soft.
My 86, 85 boats are very solid but I consider them fraggle. Thinner and although not soft, they do creak and crack. I do handle them with care and I don't take them off shore if I have another boat to sail. Just my preference. These boats have lasted 30 years and I won't be the one to ruin a good thing.
I have also had an 84 just like yours. Black glue, mfg early 84 and was totally puzzling to me. I could probably find the serial # if you're interested.
I weighed the boat and it was 25 lbs lighter than the older boats. So, my deduction was that it was a light hulled boat (before Hobie Changed the glue).
Why do I care about the weight........because I race them and I already give away to much weight in body(s) weight.
SRM is correct that if you do the technical improvements to the boat you should be fine............