Lots of good advice above. (If you are looking at a Kubota, send the S/N to them via email and they will tell you when it was built.) I have had great luck with Craig's list, but then, I lurk and look for however long it takes to find what I think I want, and I have a tractor (which I will sell, when I get a new one) in the meantime. For some reason, I am wary of eBay equipment.
Don't worry about OEM lighting- as you will find out from here on TbN, adding LED work lighting is cheap and easy.
I would post answers to the questions from #4 to get more detailed help here. And, look at Tractorhouse.com to find lots of tractors in all price ranges, just to get an idea of what whatever you consider is selling for (well, asking prices are mostly higher than final selling prices.)
As for hours, one of the best tractors I have had came with 1200 hours, but it had been maintained nicely, and I had zero issues with it while I had it.