Hi mate, and another welcome to another Yaris owner
I've had mine for about 3yrs now, and put a fair few mods on it so far, but there are plenty of things that you can have done now.
I've had the Toyota kit put on mine which looks pretty good, comprising of side and front skirts; already has rear TTE spoiler (standard on SR model); I've had aftermarket twin headlamp conversion; aftermarket rear lexus-style lights too.
Alloys on the Yaris: you can get away with 17's if you have the right tyres on them (low enough profile), although am not sure on the numbers myself!
Mine are 15's which I think are fine, although 16's is probably as big as I would go if ever I changed (my preference, but there are a few guys who've gone for 17's and they look pretty darn good!)
Any more questions, then feel free to ask me, and I'll do my best to help
Paul