Dennis,

SCHRS has a rating for a Dart 18 fitted out with a spinnaker check it out on www.schrs.com, so you can race on handicap. On the other hand, to compete in class events you can't carry a spi. I don't know how much of your spi gear you have to remove to be class legal, i'd say check it out with the class org. I'd be inclined to take it all off when racing class events, if you can't use it, why lug the weight around?
I understand that it's daunting task to buy a cat on a limited budget, I've been there and still am running my trusty old P19 on a shoestring, but once you accept your limitations you'll start solving problems instead of looking for them.
I honestly think you're going to struggle to find a cat that meets all your requirements at the budget you stated, so stop worrying. Unless your name is Bundock, Ashby, Brouwer, van Heemskerk, Booth et al you are going to be the limiting factor on your boat. I'd say pick a boat that fits your circumstances most closely and start having fun while learning to sail the tits off it!