I say purely because of the extent you are going to strop up the engine go with the s13, it hardly matters if its got 200,000+ k's on the motor, its going to be an all new motor by the time your finished!
Its Cheaper too.
If you have a change of heart and get cold sweats thinking about how much it will cost for all that and want a stock motor, do what dattodude says and get a s15 one for all the reasons he said.
my 2c

Nick