Wireless Home Automation 2.4GHz versus 433MHz
Putting this in an answer just because it's easier to type here. I did a similar automation project at home like you describe, so I'll share how I felt when the project was finished.  I did custom boards with a TI MSP430 @ 900Mhz, just point to point with a MSP430 with ethernet as the access point.  I wish I had picked 433Mhz, I expect it's range would have been better for me. 

I picked 900Mhz, like you I was looking for something that was not in 2.4Ghz which I figured would be crowded with wifi and cordless phones.  I wanted a small antenna though so I chose 900 over 433.  Using my little TI SA there were other 900Mhz sources in my neighborhood but it wasn't crowded.

There are definitely power restrictions, and transmission time restrictions in all those bands.  TI has a nice app note about regulatory compliance in these ISM bands.  Most interesting to note is that if you do frequency hopping you'll be allowed to use more power than if you just pick a channel and stick with it.  That's a little more work, and for me it had battery life implications.  My nodes sleep and poll to look for commands from my phone.  If I used hopping they'd have to wake up and find the AP's frequency first which burns more power.

I think if you look at some of the micro controller solutions you can find pretty cheap 433Mhz transceivers.   That was another reason for 900.

Ah for your other questions about interference it depends, certainly it would be easier to design a system where you always had access to your channel and no one else interfered.  Knowing you have to deal with interference will cause you to handle things like re-transmissions, acknowledgments that messages have been sent, maybe error checking like CRC, and collision hold off.   

Really it's not a bad thing, you will end up  making a more robust product.  You don't want to send your switch a signal to turn on and have it never get there.  Better to expect interference and make your system robust enough to try again.

Repeaters can definitely work, people use them all the time.  To me a repeater infers that it will be on all the time so not good for a battery powered EP, but there are other mesh networking strategies that could work too.  I'd start with just point to point first to get yourself going, but you can always dive right in.

Just my 2 cents, good luck with the project.