Yes, you can set up a tent (once you've found one) and "save" it so the tent (and its content) stays on the server (but only that specific server) when you login again or even after you've been killed. A tent has a number of slots (just like the personal backpacks) that can be used to store stuff. Only the player who set up the tent can break it down and take it again, but anyone can access the content. So it's a good idea to hide the tent in a remote place. 225^2 km do provide for a lot of space, thankfully. And yes, you can repair (and use) cars and helicopter, if you have found the right parts to repair them.
"...late afternoon the Air Tasking Order came in [and] we found the A-10 part and we said, "We are going where!? We are doing what!?"
Capt. Todd Sheehy, Hog pilot, on receiving orders during Operation Desert Storm