Hi,
For wheels, remember that certain sizes dictate specific tyres. Go for something easily available both for street tyres or semi slicks (if you go that path).
18" is a good size. Width is completely up to you.
For exhaust, make sure you try and get something legal. Saves you in the long run. As for dumps, read up on split vs bell shaped dump pipes.
For pods, theres a huge selection. Justjap now has sheilds in aluminium and carbo for reasonable prices.
I'm assuming by boost gauge you're referring to boost controller. Plenty out there now (Turbosmart and HKS now have street models).
As for ECU, Power FC has always been a standard but there are others now depending on long term goals. Look around at ViPec, Haltech and even Nistune.