Yes, the lithium AA's are about half the weight of the alkalines.
With respect to cosmetic condition, I've had a couple F3's where they were very nice cosmetically, but had mechanical/electrical issues. Both were bought locally (tend to prefer that, since I can then test stuff out before I make a purchase). One had an exposure compensation dial that wouldn't lock at "0", the other had a battery drain. Would kill off a CR 1/3N within two weeks, and, if a MD-4 was attached, would run the AA's down to the point where only one LED would light on the battery check in the same amount of time. Returned both cameras. The two ugliest F3's I've owned were both the most reliable. One was my first F3, which was the P version. Had major brassing going on, but worked like new. The other was an F3 eyelevel with the same amount of brassing, a dent on the prism, and a cracked frame counter window. Again, it functioned like new. I now have a cosmetically nicer F3P than my original, but, it needs work. If I could find out where my original body wound up, I'd buy it back in a heartbeat.
The same experience has applied with most of the other Nikon bodies I've owned. The uglier the body, usually the better it works. There have been a couple exceptions for sure. My two ugliest FM2n's both had meter issues. One had a very twitchy meter display, the other had a nice and smooth meter, but was off by a stop or so. Bought a parts body for the first one, so it could be overhauled, then with the other one, had the shop recalibrate it.