and your subfloor needs to be 1 1/4" thick to take tile.
concrete scratch can be foregone if you use patch leveler on a good solid subfloor. I recommend a latex modified patch leveler to level out the surface of the floor. You're going for a subfloor that does not move AT ALL and is as close to perfectly level as you can possibly get it. Don't skimp on the prepwork or your tile will be wrecked.
I second the suggestion to get spacers. Use 'em. Your tile will look like crap without 'em.
When you lay the tile, let it sit for at least 24 hours, then grout it. Grout a small section of the floor at a time, and come right behind it with your wet grout sponge and get the excess off the tile - if you wait too long you'll have a nasty film on your tile that's VERY hard to get off. Be sure to use sanded grout - unsanded grout is not ideal for a floor, especially if people will be walking across it with snowy shoes. And use a grout float. Lots of people think they can get the grout in there with their finger, and they can, but it tears up your finger. I've seen some groutwork with blood mixed in it 'cause they used their finger
Seal the grout as soon as it's cured. If you don't, the grout will look like crap in short order.
Black and Decker publishes a very good floorcoverings how-to book. I would strongly recommend buying it before you start this project.