There are far too many options to cover here Trev, but to give you a condensed answer; you can buy basic forks for around $400 or spend a wad which would exceed the price of a new bike. The after-market price on forks is big!
The cost of service & repairs depends on the fork and what needs to be done, but to randomly pick a figure, say $150 - 200.
You don't mention the condition of the cassette, chain, chain rings, bottom bracket, wheels and bearings, derailleurs or cables, but unless they've already been replaced, there's a more than fair chance some would need to be replaced now.
A full strip, inspection and re-assemble costs $150 plus parts if needed.
Nowadays $700 buys you an average MTB, but nothing flash, so it depends what sort of riding you'll be doing. If you hope to do any real single trail or downhill, you'll be found wanting, but if you just do rail trails and shared trails, fire tracks etc you might be satisfied with one in that bracket.
Life is too short to bother with idiots