I've seen several of these for sale on here but I don't know much about them and don't want to end up getting burnt. What do I need to look for when considering a used unit. I know they run around 379.00 new and I guess that price is without custom tunes. If I do buy a used x3 what will I have to do to make it work on my truck. And what would a good price be that would make it worth buying used.