I'd rather not use wireless, as wires are required anyway for powering devices and as the device is supposed to be a security system --
i2c bus extenders sound most interesting in here. Also this device is pretty expensive as most parts have their own arduino and displays + ttl cameras + fingerprint scanner + touch screens aren't cheap, I think ethernet - as expensive device - should also be out. Also I think if eth/wifi would be used - system could be compromised..
XBee - I think it maybe won't have long enough range. In the open it might work, but here we have walls.. For wireless keyboard it's propably fine as you can carry it to where ever there's signal..
1-wire sounds interesting; but I am not sure if if's suitable for my needs; here' another thread: http://arduino.cc/forum/index.php/topic,19707.0.html
Also - I don't know about CANBUS.. Googling it showed me a list of applications about connecting arduino to car's obd socket..
RS-485: That sounds perfect. But also a bit pricy - and impossible - most documents I found about rs-485 and arduino say that I need a sensor shield. Well I cannot install a tft01 shield over a sensor shield, actually, sensorshield v6 might allow that. Just don't know how many pins it needs.. Maybe I can provide a COM port without sensor shield.. Also rs-485 boards are a bit pricy, what about this board, is that any use? UART Serial to RS-485 Module (MAX485, RS485) – ElectroDragon Atleast it would be pretty cheap..
So thanks for some more ideas, rs-485 sounds about right but needs further investigation.. What do you think about the project in large scale?
BTW; here's a good site about arduino and rs-485: Error, Electronic & Electronics Components Depot United States
This is even more informative page about arduino and rs-485: https://arduino-info.wikispaces.com/RS485-Brick