Before you can put down tile, you need to find out what layers there are on the subflooring, and the condition, size, and spacing of the joists along with their unsupported length. This has little correlation to the room size, it is the total length of the joists that can deflect. A cbu on the floor does NOT appreciably affect the structure strength, so unless you want the additional height, 1/4" stuff is fine. Choose the brand you prefer. An alternative is to use a membrane such as Ditra from www.schluter.com or another brand. This is easier and faster to install, and actually does a better job.
Suggest you check out www.johnbridge.com for your tiling questions. They have a deflection calculator that can tell you if your floor is suitable for tile. Also note, the deflection requirements are different depending on whether the tile is ceramic or natural stone (porcelain is a type of ceramic, and doesn't get its own category). There are also very detailed instructions in their "liberry" on the correct method to build a shower. Yes, you need a liner, and yes, you (typically) need a preslope. Forget greenboard...use cbu. You can actually put normal drywall on the walls if you use Kerdi, also from www.schluter.com (go to their site and watch the video to see how it goes together).
Doing the whole thing in a week is optomistic, unless you are a pro. It almost always takes longer than you think, and the older the house is, the more you find needs to be repaired in the process. Cast iron stuff is harder to change than pvc. If you are going to change the tub to a shower, your drain line needs to be upgraded to a 2" from what is likely 1.5", too. So, good luck and ask some more specific questions.