You can fit 235/40/17's on the back of a 2wd saph and they do make a hell of a difference. Just get used to the car and if you still want to go 4wd and want to keep the car you will need, the following bits off a 4x4:

Gear box and prop
Front diff, drive shafts, hubs, and sump.
Clutch cable different aswell i think.

I know you can use to 2wd rear diff but not sure if the ratios are the same so probably easier to go for a 4x4 one instead.