Regarding old gas lines... When I first set up my kegerator with parts that my dad gave me, I re-used the 20+ yr old gas lines. Worked fine for ~6 months. I only replaced it for the heck of it. Probably would've worked fine for another 20 years

. Still a good idea to replace the lines though, they don't cost much.
Most kegs come (slightly) pressurized as far as I know. Just pull the relief valve on the lid until it's depressurized. It may still have old soda syrup in it, so give it a good cleaning.
You're well on your way to a homebrew kegerator set-up!
