Salt is nothing to be afraid of.
I have always run my skis in salt and as long as you flush the motor and spray the bilge (motor and everything in there) with a good spray like Boeshield T-9 you will get years of trouble free riding. I'm running a 19 year old electrical system on my square and never have a problem. It has been sunk many times and always starts once dry.
Do your self a favor and look at this site
http://www.x-h2o.com/index.php
Check out the classifieds and look for deals.
Squares pop up from time to time and you can find one with a 701 in your price range. Even a 650 SJ with a few tricks can be a fun ride and can be a blast in the surf. Just about everyone who starts off with an older Kawasaki sooner or later sees the light and gets a Superjet. It's not all hype, so try to get the best platform for your money.
A few people from your area post on there and every so often we get together at Sebastian. I'm sure they can help you find something decent and get it up and running.
Bottom line, get a square nose (1990-1992 650 6m6 motor, 1992-1994 701 61X motor) in running condition. If you look hard enough you might even find a round nose (1996 and up 701 62T motor). Most prefer the RN (round nose) over the SN (square nose) for the look but hulls are identical so why pay more for the newer top deck.
If it already has the popular mods (pipe, prop, shortened handle pole, footholds,..) all the better.
If you find one that's all stock than don't worry. The gang over at X-h2o will help you get the most out of it for little $$$.
I'm rambling on a bit but unlike car audio, I do know jetskis.
Feel free to PM me here or on the "x" if you need any help at all.