Ok, chances are your tire size is 255/70R16. 18s would be a much better choice simply because there is less to worry about.
You would be able to just bolt them right on with no trouble as far as rubbing, provided you go with a smaller sidewall. I'd go with something around a 255/50R18. 255 is the width of the tire. 255mm across. 50 is the a percentage. in this case, it would be 50% of 255mm. This gives you the height of your sidewall. R18 is obviously the rim size. 18". Ok, now the the teaching is over, with something like this, you wouldn't run into any rubbing issues. Tires would still be expensive, from 300-500 each depending again on brand and quality. I just look up the size on www.tirerack.com
and found 2 pirellis on there. $370 to $393 each. And again, you'd have to have someone put it on the wheels for you.
As far as your brakes go, stock SHOULD be ok. and you won't lose too much power/mpg with 18s. It will stop slower with stock brakes, but it should be ok. Just have to get used to it.
The speedometer WILL be off no matter what size you go with, unless you stay at stock 16". The truck was set up for that size from the factory. I've heard that some techs at shops are able to calibrate it to accept the bigger size. I don't personally know, since I buy aftermarket wheels of the same size as stock.
All in all, look at spending about 1500 for a decent set of tires, installed and balanced, when you need new brakes, get the higher end brakes (Spend a little extra to stay safe. It's ALWAYS worth it), and have someone adjust the speedometer for you.
Hope that helps you!