Anything I should look out for while fitting them?
Kids going past doing this....
Seriously though they will slow the car down and make it handle worse due to the extra weight.
I was thinking of getting some nice road wheels in 17", do they fit alright with coilovers? Anything I should look out for while fitting them?
People who say you dont wanna go any bigger than 15s have no clue at all.
Why do you think that mate? My old mr2 had 17's, but just before I sold it I put original 15's on, and was amazed at how much nicer it was too drive! Big wheels are for posers IMO, for driving, small wheels are better in everyway, thats why toyota fitted them as standard.
Have you ever tried a lightweight set of 16s? practically no difference in weight to say a normal set of 15s.