. . . but at what time does it not make sense to keep putting money onto such an old boat when i could of had something newer for a little bit more?
Do you want the rational financial advice, or the labor of love advice? Do you want to race or just have a good time?
The 18 is no longer produced and later models are hard to come by, and there are advantages and disadvantages to an 18 and an F18, depending on what kind of sailor you are. The 18 is a very good recreational boat and the F18 is better for racing.
1. Bullet proof. The 18 is a strong boat with thick hulls which can handle being dragged across the beach and clumsy crew.
2. Forgiving. Mistakes seem to be absorbed by the boat rather than amplified.
3. Still a boat-load of fun compared to about 99% of the boats out there.
4. Very good availability of parts.
1. Older design which is a little slower than modern designs.
2. (Maybe hobie18rich can fill in the rest)