First, as others have said, don't assume that you need more power. Add one thing at a time and see how it goes. If you aren't running lots of locos and distribute power well thru your wiring, you may find the Zephyr has all the power you need. If not it's easy enough to add a booster.
If you're going to go handheld, the UT-4 won't do accessories like turnouts so you will need one of the more expensive ones. You can add receptacles easily enough, they just all plug together with cables.
Decide ahead of time whether you want to do radio control or not. If you do, go ahead and get a handheld with radio in it, even if you aren't going to be adding radio for a while. However, note that infra-red may work better than you think. I'm experimenting with my UT-4 and only one infra-red receiver and am surprised at how well it works. Adding several receivers I suspect will make it work very well, and cost much less than adding a radio receiver (or two).