Sounds like all you need is a
Store plugin, then just change the currency from the usual "Credits" to "XP"?
It may have much more than you actually need, but you can just deactivate / remove the unnecessary features.
As such, there are no good reasons for re-inventing the wheel, as the only thing you require is a different currency name.
The downloads can easily be solved with
SM File/Folder Downloader and Precacher, where you just specify the folders with your custom files, and it'll automatically add the files from there.
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