Good for you to start thinking about JK again.
Fuel economy
- second to none! thirsty wise.. without any mods, 12~13l/100. with mods, it'll sky rocket. mine is an overland with front and rear bar done and a 2.5" lift. The average is already 15~16l/100. By the time roof rack, tire carrier etc.. are added, I'm expecting 18l/100. I'm light foot on the pedal by the way. Most of the time I see the "eco" light on. lol
Servicing
Depends on what you do. If the JK not going to where it should be, then any dealer is fine. If it's going to live in its territory, get it serviced from a JK specialist. Servicing from dealer is not that expensive which average out to be around 400~700 for petrol.
Parts
Again that depends on what you want to do and if you like JK's behavior. Since you owned TJ before, you know what it is like anyway. For me, I would say most can be retained except for the suspension. It's just not my cup of tea from stock.
And if you are looking to gain more clearance, suspension system and tires are the first things to go anyway.
Life in city
It won't be easier than defender for sure. If you have all the bar works done, the good thing is that those mini cars will stay away from you. If you have a lift, make sure you can fit into those car parks. Another thing is the blind spot problem, there are simply too many blind spots for a lifted JK. Sometimes you can even see there is a sport car next to you. grins.
Customize ability
Take your bet. Choose what you like. If you can't find it here, you can always get it from US or Africa. It's a most tiring task in owning a JK. Too many mods, too little time and not enough money.