Put together your build first, then see what PP you can run it all on. If you're a couple of % over even with cargo hatch and whatever disabled, see if you can drop something or lower something's class to get the next down PP, provided you stick with A class for the heat efficiency.
Heat sinks...are not as great as I hoped they'd be for smuggling. To be honest if you sit outside your station for a few seconds and turn the power hungry modules off, you have plenty of time to go SR and get in the station. The only time I see them being useful is if you want to play Open, have the balls to run from a player pirate interdiction and want to use SR to lower gimbal effectiveness. But truth be told silent running isn't as "disappear off the face of the galaxy" as you'd want; you'll still get hit by anyone with a half decent shot.
Cobras are certainly not a bad ship to start in though. They have a really tasty speed among anything else, so with control you get into that station quicker and can be a pain to hit after submitting to an interdiction if you do choose to run. Just remember not to boost in a straight line.
Edit: here's an example build that retains an edge on survivability:
http://coriolis.io/outfit/cobra_mk_iii/02A4A4A3D2D3D4C18---00-03034801012o.AwRj4yKA.Aw1-kA==
Drop the shield for cargo room if you want to go hardcore, and if you want to us the spare utility slot, heat sinks are still not a negative thing.
