How much should it cost to make my Impreza look cool with aftermarket parts

You cannot make an Impreza look cool with after market parts. You will make it look like a tarted up rally car at the very best..

If you go to OTT with the after market parts you will turn it into a chav car and destroy what the car really is. Beefing up the engine on the other hand . GO FOR MY SON :P

Oh yeah to get a higher rated fuel pump to tweek your turbo will cost around £500-£600, That would be the cheapest and best option.

Body kits for anything half decnet you could be looking upwards of the £2000-£3000 mark.

depends on what you call cool. probably just some rims an lower it a little. if you choose the right parts and plan ahead you will save your self a lot of time and money.
check out these sites, they may help.
cardomain.com
andysautosport.com