Here is what I did :
EngineSystemsManagement
My video in action.
http://youtu.be/yFyMZ4urR9U
Tach is not so easy to do, its a VR not a hall effect sensor. My only real concern is overvoltage. I've had the tach plug hanging out of the cluster for a while now wiating for some voltage readings and just haven't had time/interest. Too many house chores.
Using the stock boost sensor is def the way to go IMO. Be careful of ground looping your EGT. I spent days cursing because it would work when powered off the laptop but not the engine. Like you I had high hopes of using my android for a display using BT for comm, but at the end of it all an LCD was way easier and more sensible. I still have the BT shield, probably going to use it to make a nest like tstat for the house.
I don't think your choice of arduino's is great, not nearly enough pins.