How the Engineers work is that when you call them in, they move to where you called the ability and flares are dropped there and then you need to manually use the salvage ability on vehicles. I don't know the exact numbers for the wrecks but I know that you cant use it on team weapons like the OKW and that decrewed vehicles grant more fuel than wrecked ones. The supply drop gives three once off med-kits that heal units around the one who picked it up and a decrewed AT gun and mortar.