"The z21 looks good, but it seems to have a European style plug. Will it work in the US?"
z21 works just fine with American trains. I've been using the system for about 18 months, hands down it's the best model purchase I ever made.
I would suggest that you buy the "z21 Start" box (the white box with the little "z") instead of the considerably-more-expensive Z21 (black box with the uppercase "Z"). I've found that the little one has all the functionality I'll ever need.
If you look on ebay for these, there are Euro sellers selling them in the $100 range, with low-cost shipping. You'll also need to buy the dedicated modem and key code to unlock it (these are available from USA sellers).
You may need the 2-prong "Euro plug adapters" for the power supply.
I bought a separate power supply :
The particular one I bought was the PST-YD100W
If you have a smartphone or tablet you can try the z21 software RIGHT NOW.
You don't actually need the z21 (although it's just running in demo mode, you can't actually control a train with it without "the box").
Go to the Mac App Store or Google play and search for "z21".
Then give it a try.