Hey, welcome.
Sorry to keep you waiting, the holidays have been crazy haha.
Anyway, I'll try to address your points piece by piece here.
Wheels: That's a decent size. 18" is probably the max you should comfortably run on the Celica. Consider the fact that you need low profile tires as well. However, you also mentioned that you wanted to go wide, so it may be worth it to get a wider wheel set up while you're at it. 8.5" is bigger than stock, but still barely flush with the body line.
Exhaust: Being in the US, I can't tell you much about a local expert, I can only give you advice. Consider a TRD exhaust if one comes up near you. Best for sound and power. As for deep, GReddy, HKS, Tanabe, and Invidia all make deep sounds.
Spoiler: Again, the TRD Action Package spoiler could be the move here. Very tall. But if you want to stand out from other Celicas, there are a variety of other options. Check out CarID.com for some ideas. You could buy a name brand or take a chance on eBay with a replica.
Color Scheme: Can't go wrong with black as the secondary color on any other color. I'm sure it'll look good if you're not sloppy.
Price: I always prefer to buy used if possible, but I'm cheap. Here in the states, you could pick up a decent USED wing, wheels, exhaust, and lights for maybe $800 USD. New would be more. Could also be less if you're resourceful.
I've been able to make my money go a long way with these cars by being patient and smart. Don't rush it.
Good luck!