Start by reading through this: http://www.tacomaworld.com/forum/sus...uide-read.html
The kit itself won't necessarily lower your MPG. Yes, it gets the front of the truck up in the air a little more, and that allows more air to flow underneath - potentially increasing your drag. That increase in drag, I would think, is negligible. People see reduced MPG with lifts because they typically run bigger wheels/tires with the lift. These new wheels/tires are usually a lot heavier than the OEM setup, and that weight is more rotational mass the engine has to work against to propel the truck (larger load on the engine = lower MPG).
As far as spacer lifts ruining things, you're only preloading the factory springs in order to achieve the lift. The geometry of other components remains the same (the rest of the suspension doesn't know if you're running an aftermarket lengthened coilover or a preloaded stock coilover). The only things you run the risk of wearing out early with an in-coil spacer lift are the coil springs since they're under more pressure/preload. On the other hand, a top-coil spacer like ReadyLift does not prematurely wear out the coil springs because it sits on top of the strut/coil assembly. I've read posts about top-coil spacers potentially wearing out ball joints due to extended down-travel when the suspension is unloaded, but I've never actually heard of/seen anyone actually experience this.
I am by no means a lift expert, but I researched the same things you're asking prior to installing my mild lift & tires. If I'm wrong with any of this info - please, someone chime in!