The JC2-MP buy menu script implements a fully-featured buy menu within the game, allowing you to purchase vehicles, weapons and skins. This script ran in the JC2-MP public beta tests, and has been tested significantly.
Edit shared/buymenu.lua - inside, you will find lists of vehicles, weapons and skins. Edit these as appropriate to add and modify the items you want. The DLC items have been commented out; you can easily re-add these if you would like to use them on your server.