i just put a turbo on my car. I went with the GReddy kit since its 50 state legal here in the US and a pretty simple and straight forward install. I didn't have to upgrade anything for the kit. It pushes 5.5 psi for the boost. I love the difference it made.
As for pre-turbo checklist, I was just like you in with the questions, and I still am (right thermal?
). The only thing you need to upgrade with the GReddy kit is a more open exhaust. The stock piping is very restrictive and with a forced induction setup it will hurt you instead of help. Get an exhaust system with at least 2.25" piping if you plan on staying in the 5-8 psi. Anything higher than that I would go with 3" piping. I'm going to replace my exhaust piping with 3" all the way once I turn up the boost.
One of the biggest things overlooked before turboing is a regular engine tune-up. Check everything out before you slap on the turbo. Plugs, wires, fuel filter, compression, etc... all that will help you in the long run. And plus it won't hurt your engine either.
Like I said before, the only upgrade I had to do before turbo was my exhaust and I at the pre-set boost of 5.5 psi. Now before I turn up the boost, I will have to change out plugs, fuel pump, injectors and all that stuff. I'm not going to replace internals because 1) I'm not going to boost more than 10, and 2) I'm going to have it professionally tuned. TUNING is the biggest key when it comes to the turbo. Remember that.
So sorry for the long post, but I would suggest the GReddy kit as for the sake of simplicity. Before the turbo, perform a complete engine tuneup and upgrade your exhaust. You should be in goods hands from there.
Oh there is another post by Dr. Drew on what setups you should run according to turbo applications. Read it!