do i need an external wastegate with this turbo or can i use the internal one?

 

You dont need to run an external gate with that turbo because its internal gate. If you wanted to run external gate you could weld the door closed, and get a manifold made for ext gate.

 

Can I use a 3 in. downpipe? and keep a/c?

 

Depends on manifold, but its possible.
what car? what engine?

 

a 2000 si b16a2 was planing on going with the hks mani

 

Check the HKS website and see if they have a downpipe to fit the turbo and manifold combination. I highly higly doubt that they would have all that without it being a complete kit. Some logs are A/C compatible, and some aren't. If it comes down to it, you can have a DP fabricated.