I just got my 18 about a month and a half ago, and I have experienced similar issues/had similar questions. I'd just like to add:
When it comes to hoisting the main, some lube helps. I've read either dry silicon spray or even dish soap helps. Silicon made a world of difference for me. Also, check the sheaves that the halyard goes around. One of mine wasn't rotating, and 75% of it had worn away on one side! I've read that if it goes up hard but comes down easy, a smaller halyard might be the trick. Haven't tried it.
If you're looking to replace parts, lines, or even try to figure out if you have all the pieces and how they go together, get a sailing parts & accessories catalog (free from Hobie: http://www.hobiecat.com/support/catalog-request/
) and study the Hobie 18 parts breakdown.