I would second what Trey says. I was thinking about the cost if you were to use Formula 18 parts.
$900 Spinnaker
$200 Pole
$300 Snuffer system
$200 Blocks
$ 50 Cleat (halyard)
$100 Sheet
$ 60 Halyard line
1810 Total

Estimate that does not include the labor and thinking time.

Hobie 18 worth $1800?

I am thinking that is about $3600 towards a Formula 18. You can pick up (these mostly in catsailor ads):
Inter 20 $7950
2001 Tiger $8500
Tiger $7500
These may not be available at this moment but you get the point.

I cannot believe you sold a Fox and are thinking about making a Hobie 18 faster by putting a spinnaker on it. The Fox was a good boat! One of the Fox owners on this coast put a large main, equaling the I-20, and it was an awesome boat! Just as fast as the Inter. You would have more boats to race against having done that, as well. Who can you race as spinnaker Hobie 18 against? Put out a little more money and get a Formula 18. It will have resale as well.

Later,
Dan