Running 32s and stock gearing will knock about 5-6 MPGs off at least
Then like mentioned will run hotter since the transmission is working harder trying to flip back and forth between gears
Of course that leads to being harder on the transmission itself
Power on hills or even small grades makes it difficult to maintain an even speed even on Interstates
The list can go on, but its one of those cans of worms that some open, other take a look at the label and shy away
As far as to what kit to buy it all depends on what you have to spend and the height you want to go.
I will tell you after running two different OME setups/ The Ironman setup for a while and now the adjustable coilovers, all I can say is " I love the coil-overs for the ride and adjust-ability of them"
But the OME or Ironman both ride and handle a 100 times better than stock