Pole hole spacing in the covers should be the same, and all based on the Gibson spacing (but you never know). The thing to watch out for is if the kit P-90, being a low-cost Chinese version, uses a different construction method in order to cut costs. You might not be able to change the base-plate over easily as it could be more of an all-in-one construction. You won't know until you get it.
As it's easy to change a single dog-ear P-90 over, I'd recommend trying the kit P-90 first, just to get an idea of what it sounds like.
You can get very low-cost P-90 dog ears from Warman (which may not be that different from the kit pickup), lowish cost Kent Armstrong P90 dog ears and nickel covered Wilkinson P90 dog ears. After that you move into the standard makes like Seymour Duncan, Gibson etc, and then the more boutique makes.
If you like the way the guitar plays and feels after it's put together, then as it's only one pickup, then it may be worth spending a bit more to make that a good pickup. A good brand pup should also be able to be sold-on at a good price on eBay if you ever get rid of the guitar - after re-fitting the original (which you will of course keep) - as you will never get as much for it installed as selling it separately.