hi mate,
all the welding stuff you asked is right. the injector bosses hold the CA18det injectors and yes, these come with it.
all the welding will have to be done by your local tig welder (unless you have one!) but the exhaust stuff can be done with any garden variety mig or arc welder.
the megasquirt computer is an aftermarket kit computer that uses a MAP sensor (manifold pressure) as its main sensor for injecting the fuel correctly. i have already installed this in the curcuit board. it is good for 20 psi i think
the other sensors you might need off the top of my head are an air temp and a water temp sensor they will cost you next to nothing. as the computer is made for te home tuner, they even tell you what sensors to use from other cars you can find at the wreckers! the throttle position sensor is on the throttle body and i think thats it.
heaps of info on the website
megasquirt