You have to decide the "granularity" of the system, at one end you have a single controller and 50 wires running to sensors around the house. This model is not appropriate IMO.
At the other end you have 50 tiny sensors connected to a bus of some kind. To do this the sensors have to be intelligent and you need a protocol to get readings from each sensor.
In the middle is what i suspect you are suggesting, an Arduino in every room reading 5-10 sensors and a network for them to talk to a central controller.
How do I hook sensors to the board
Most sensors just connect to an analogue input, but many can talk I2C or SPI.
does each one has to be connected individually or I can use a "bus" wire
Either way works, but if you use a bus you have to design some form of protocol so they don't "interfere with each other".
What's the maximum/suggested length of the cables to the sensors
Analogue sensors are prone to noise so long lines should be avoided. But it depends on the accuracy you need.
Are there only analogue sensors?
Most sensors read an analogue event, some report the results as an analogue value, some use digital techniques to report the value to a processor.
Are there digital T sensors for example?
Don't know what a T sensor is. (temperature?) But there are sensors for everything you can imagine and then some.
Are digital sensors "better" than the other type?
If by "digital" you mead the reporting mechanism, IMO yes that's better. Convert analogue to digital as soon as possible.
How do I hunt for compatible sensors?
Just start googling I suppose. Look for "temperature sensor" etc. Or ask here for a specific thing. Most sensors are simple to interface with, normally just needing an analogRead() followed by a small amount of maths to scale the reading. Others use the standard digital comms like I2C and SPI, there are existing libraries to handle that.
A few are really difficult, I would suggest you ask here and link to the data sheet before buying anything.
Whatever you do I would plan to run some Cat5/6 around the house to every nook and cranny, it's cheap and easy to do during the build, damn hard after.
Rob