As others said, the progressiveness is built in to the stock suspension linkage. If you look at where the bottom of the shock attaches, it isn't bolted to the swingarm, it's bolted to a little network of levers. The levers are there to create the progression.
The stock springs will work. Most people never touch them. A stiffer front end will help most on downhills and in deep sand. A stiffer back end will help with a heavy load. It's also important to keep the springs balanced- being soft on one end and stiff on the other is probably worse than being soft on both ends.
Since you're on a budget, I'd say start with the stock springs, load up the bike and try to ride it in as many different conditions as you can. If you don't like how it handles in a given situation, fix it. Get all the free adjustments (preload and damping adjusters) dialed in, then if that isn't good enough start changing parts.