So you need:

- 18 to 20 feet. You will not fit 3 to 4 adults on anything smaller
- Fiberglass. Lighter and faster than any rotomolded boat.
- Center board or Daggerboards a must. Makes for a better boat that is easier to tack and goes upwind better
- Not a buoy only toy, aka extra light, extra powerful. You need something more substantial.

In the $15000 price range, go used. I'm not sure what you can find locally but in Europe with those constraints you can get:

* a Hurricane 5.9. Really nice boat, the version with a pinhead main and no spi is a great reaching machine.
* an oldish F18 (like the Tiger, Alado, Mattia, Cirrus, lots to choose from)
* An old Tornado could do the trick on the extreme end of the scale. Those are *big* and powerful but you will not find anything faster! Price of those has gone down recently, but maintenance costs are heinous however...
* A Nacra Inter 20. Or its successor the Nacra 20.
* A little smaller, the Nacra 580. Not a lot of them around however. I've never seen one I think...

Those boats mostly come with spinnaker these days, or can be equipped with one. You can remove the spi hardware and sail them without it, but frankly that would be a shame. Spi are mounted into a sock and go up in three seconds on any tack. I can manage steering and the spi on my F16 sailing solo, no problem. It just takes a bit of training.

I'm sure people on your continent will have good suggestions as mine are probably more tailored to Europe: I'm not sure there are any H5.9 in North America. A shame really as it would be the perfect boat for you