1) In terms of grams produced per pound spent after a few grows hydro will be a much better investment. Most of the equipment (ph meter, ec meter, tank) can be reused. I used to grow in soil but would never think of going back now. Hydro is QUICKER, EASIER and has a MUCH higher yield.
2) I grow 4 plants in the space where I used to grow 5 so slightly more space needed for hydro but this varies depending which method you use (DWC, e&f, NFT)
3) Hydro by far
4) Pretty much the same except you'll need a tank to store spare water in hydro but you'll have shitloads of soil to dispose of at the end of each soil grow

hope that helps man