1. Say, I wanted to handle around 50 sensors - temp, motion, light, humidity and so on - they all don't have to hook to a single board, I guess I could use a few Arduino Uno + Ethernet shield setup so they each sense their near area and push data to the server over the network.
You could go for the above mentioned or you could use 2 arduinos and Multiplexers, you'll have to search for the appropriate muxes. Mux is a device that switches between sensors so you can hook up say a temp sensor and a light sensor to one analog pin and have them switch between each other. Rest depends entirely upto your imagination.
How do I hook sensors to the board - does each one has to be connected individually or I can use a "bus" wire (I am sorry for using wrong terminology here - don't know how it's called) so they all sort of connect to it in parallel?
Parallel connections in this case would mean switching.
Will the board be able to figure which sensor is sending and will not the signals interfere with each other?
The data being sent by arduino is pure bits and bytes. So you have to label that in such a way that you can differentiate between what is being streamed. As far as signals interference is concerned, only one type of data is being sent over once and that too a separate channel. So say as mentioned earlier you're using a temp sensor and light sensor you have a variable
int temp = analogRead(temp_pin);
int light = analogRead(light_pin);
// note that light_pin and temp_pin are essentially the same pin, you need to differentiate at your own end as the analogRead function will just give you ADC //values, its how you trap it and process it that labels it
2. What's the maximum/suggested length of the cables to the sensors? Are there only analogue sensors? Are there digital T sensors for example? Are digital sensors "better" than the other type?
Usually sensors involving PWM need to have a short length cable, infact that goes for almost all the devices. So, in order to get an extension and do extensive wiring I would suggest using the ethernet cable. Twisted pair CAT -6 cable. Its very effective. But it won't be able to handle large amounts of current so if you have say a servo you might want to keep the supply different and the PWM signals coming from the arduino different (through the cable).
3. How do I hunt for compatible sensors? I guess you can interface pretty much any sensor but there are sensors that are just "friendlier" in terms of connecting and reading their data and don't require figuring out your own library for parsing its output at a very low level and do all sorts of math around it to get the proper number?
You just need to go about in this fashion. What kind of input does arduino take -> PWM, Analog, Serial etc. what's the voltage level on this 0-5v only. So now you have output parameters set. You can check Digikey, Mouser, Adafruit, Sparkfun, RS components for sensors having 0-5V operational range and are based on Serial or PWM or whatever you want.
common Temperature sensors are LM35 etc. So you can search other parameters similar to LM35. Its upto you really.
You can also google: temperature+Arduino, this will definitely lead to some prebuilt arduino projects involving temperature sensing etc. Remember to use the existing code than to re-invent the wheel. You can study it and reuse it. That's why its opensource, to help you.