Well I have heard many good things of the d40. I just have never even seen one IRL.

And you want THIS I want an adapter for m42 lenses. . . And I am assuming that this like majority of the adapters you can buy, will not enable autofocus. So you have to manual focus.

And after making a quick check to amazon I am also going to say that yes the XT and the XTi both can accept EF-S and EF lenses.

All canon dSLRs can accept EF lenses. And I am assuming these can accept EF-S lenses because the kit lens bundles with it, even in reviews say that the 18-55mm kit lens, is an EF-S mount lens.