Nice to see someone here after my own heart (or wallet?). I mostly replace hardware like pickups, tuners, and sometimes bridges. What you do, depends a bit on what you like. The cheapest "upgrade" might be to just swap out the ceramic magnets that the kit pickups come with for Alnico II or V (the most common "vintage" magnets). I tested the pickups that came with my kit for resistance and inductance,* and thought they might still be wound a bit hot, so I took a chance on the next cheapest option: A set of no-name "57 classic alnico II" pickups on ebay. The seller put the resistance and inductance specs up, and they were similar to the SD pickups. However, they were two conductor pickups, so I would not be able to coil-split.** That may not be a big deal with low output humbuckers since their single coil output may be really low...but I wanted to be able to experiment, and so opted for the next cheapest approach: Find a pickup importer who designs, or buys designer pickups from an Asian builder. That is what led me to Dragonfire. I have purchased from the owner before, and know him to be knowledgeable about the products he sells. I told him what I wanted and he made a recommendation. I ended up spending just under US$100 for both pickups. That's about 2x as much as the no-names and 3-4x as much as replacing magnets. There is a 4th (relatively) cheap option... Which is to buy from a small winder. This can be a nice way of getting a custom pickup for much less than SD or G prices. In the US it's still $100 to $160 per pickup, which is more than I wanted to spend, although it's been my go-to approach for singlecoils and bass pickups.
You can get Wilkinson pickups for about the same price as the no-names, or less, but they are Alnico V and hotter (I think). They seem like a good deal to me, but it depends on the sound you're looking for. A hotter PAF style alnico V pickup seems to be the most popular...and more mfgs = better prices ;-)
*My understanding is that this gives something of a comparison of the relative "hotness" of pickups that are otherwise similar to each other but the comparison is only "ballpark". I don't have the equipment to test for anything more sophisticated and probably would not understand it if I did. Please defer to Weirdbits or Doc who actually understand some things about electricity.
** Speaking of Weirdbits, somewhere here he wrote a post about how to turn a two-conductor humbucker into a 4 conductor humbucker. I actually tried this with one of my kit pickups some years ago and managed to turn it into a paperweight. My soldering skills have gotten better in the meantime, but those wires are so small! I haven't tried again ;-)