Exhaust before turbo?

ok i've checked my car and the cat is definately underneath with a pipe coming down from the engine into it, so i assume this is the downpipe?

oh, well I don't really know what size to get i was just told that 3" would be best. I'm looking to be running a lot of power eventually (300hp+), so would 3" still be too excessive? Also, does the downpipe need to be replaced with a larger one when i get the turbo?

this is probably another dumb question but does a turbo come with an exhaust manifold normally? would this include a downpipe or would i have to get that seperately?

Only thing is, i'm worried that i'll buy an exhaust and then i'll get the turbo and realise that i've got a bottleneck somewhere.

Thanks for your help