16's u have semi low profile tyres/or low prof.
17 u have even lower profile
18 is even worse
if you intend to drive the ute hard or use it as a "ute" 18" wheels dont have much of a tyre sidewall to protect the wheel if you hit a big bump/kerb/rock whatever. thats the major problem with 17 and 18" rims. you can dent them very easily if your not careful.
Also, 17's and lowering it would be a better go, unless u wanna keep it stock suspension.
Otherwise, the price of tyres .. $$$ :)