Don't buy an 8G for at least 3 years.

 

From what I hear from the inside, the 8th gens will be all sorted out by the time the Si comes out.

As far as the greatest expandability I would have to say that a 99-00 EX or Si coupe would be the best pick. If you want to swap to a B18, B20, H22, K20... Go with the EX, they're much cheaper. But, if you'd be satisfied with a B16 just get an Si.

Greatest out of the box performance, definitely wait for the 06 Si.