i would go for 31's, its a big step tbh
if you look at my post in reader rides, you can see the tj I just got its got a 4" lift and running 33x12.5x15's in the pic , as to how much it will cost for everything is a bit harder, depends on what lift you buy and what combination of wheels and tyres but I guess you will be looking to spend anywhere from £1100 upwards