I could see the building of a vehicle being lower, but just because I have all the parts doesn't mean I know how to put it together. Give all the pieces to me now, no manual, nothing, and I'll look at you like "wtf do I do?". I still think they'd need to put it say, behind lvl 3 int. It's still lvl 1 (if you do all the starting quests without leveling and put the points into int) and means you have basic knowledge of how putting those together works, even if you don't have the knowledge of how the individual pieces are built. It's kind of one of those back n forth things of how quick do they want it possible to get to "end game" right now.
As for the xp, it's kind of all over the place. Increase range of shared xp and they'll be higher than you, then the scavenger gets behind the builders. The only real way to fix it all, is to get rid of shared xp, but then that brings back whoever is best fighter gets the killing xp because they'll kill faster or headshots more. Could fix that by xp from a zombie is based on percent damage done to said zombie with final kill getting 10% bonus xp to xp that would be gained (so even if the person who did least amount of damage gets the kill, they may still not get the most xp from that zombie). There's just a lot that needs to be done and balanced, woohoo, lol.