.....4. Home security devices, I really do not know where to start......
Well, I haven't got my Linuxmce system up and running, but I am just finishing off the build of my house so I may be able to offer some recent experience.
For your points 2&4
I was at a complete loss as to what to do for security/fire sensors, but it turns out it's a lot easier than I anticipated.
I laid cat5 (actually cat6 but the principle remains) to each window location, to the corner of each room where I wanted PIR's, and to each location where I wanted smoke or rising heat sensors.
It turns out that by wiring door contacts, and PIR's as per the included instructions ( my PIR's required a 12VDC supply, that I had to send over the cat5) a simple latched contact is made when the sensor is enabled. This is a voltage free connection, so at the other end of your cable (in my case my electrical cabinet) you need something capable of sensing a voltage free contact.
In my case this was KNX devices with built in inputs. I used ActInBox Classics from Zennio
The outputs I used to turn on and off lights (I am using non-dimmable energy saving lights, so a simple on or off is fine for me - I'm also controlling some power sockets so I can turn lamps on and off to produce lighting scenes although with different KNX devices to support 13A per socket) - you use the latched output to switch the live feed to the light. For this case it's the inputs that are of interest. Simply connect the two latched wires from the PIR or magnetic contact switch to the input and you're in business (as long as your wiring lengths are not too far, in a normal house it works for me, YMMV). This produces a KNX bus signal when the contact is made or broken, much the same as an ordinary switch, and it allows you to switch anything on the KNX system. Linuxmce integrates with KNX, so my intention is to use these KNX bus signals to provide the alarm sensors in LinuxMCE. I have no experience with z-wave, but people here that it is comparable to KNX in functionality, so you may be able to do it this way using z-wave.
for me KNX was the prefered way, I wanted a wired solution, to be able to switch mains loads, and it turns out that the devices I use (Zennio are a budget KNX supplier) had very cheap inputs provided in some of their products - almost as a free extra. I'm using Z38 touch screens
for all of my room switches - each comes with four available inputs, If I had known I could have wired back to the switches and used these inputs, but as it is I wired back to the wiring cabinet and used the inputs on the witing panel.
In principle, as long as you have a simple voltage free input card/device that integrates with your computer (and is able to be seen by LinuxMCE) you can use cheap alarm sensors in this simple way (it's the inputs that can end up costing the substantial money, depending on how profligate you are with sensors).
For fire alarm sensors the prognosisis different. Fire sensors tend to come in four-wire, or in more recent times two-wire configurations. The sensors can be wired in series, and generally require an end of line (EOL) terminator. On being activated the sensor draws more current, than when in standby, so active, alarm, fault, or short circuit conditions are defined by different current draws. It's not inconceivable that you could make something to do this, but in practice a simple two zone alarm panel handles this for you and is probably so cheap that it's not economically viable to do it yourself. I paid circa £60 UKP for a 24VDC (ie a 24V supply for my sensors - make sure you match the panel and the sensors, different systems work on different voltages and, presumably, currents) two zone alarm panel with fault and alarm relays (these relays are important to me, as they latch under the appropriate conditions and I used these latched outputs in conjunction with a KNX input to provide a signal to my KNX and then LinuxMCE system - hopefully).
If you wire cat5 (cat6) to each alarm postion and use a two-wire low voltage based system (24VDC in my case) then you can couple up each set of twisted pairs (I am using green and blue along with orange and brown as my positive leads, with green/white and blue/white along with orange/white and brown/white as my negative leads). This allows you to have go and return legs wired to the diode base (in the location where the sensor resides) and you can complete the series circuit in the alarm panel. This works for me. All bets are off if you use mains voltage fire alarm sensors as these need to be wired (outward and return, or out, next sensor.....EOL) in mains cable.
So to summarise. You can use cheap door contactors to signal doors being opened (for example I use this to turn a light on in a walk-in wardrobe, and the washing machine cupboard, and the electrical cabinet), and PIR's to sense presence in a room (you would normally want to combine this with current lighting scene requirements along with how dark it is - either from a luminence sensor, or by making a judgement based on current time and date - both of which I believe can be done in Linuxmce)
I appreciate this may not be what you might be asking, but I'm telling you because it might be helpful. I had to find this stuff out for myself through trial and error, because despite extended periods of research there didn't appear to be a resource to tell me this stuff. Once you've done it, it's relatively trivial, but there are a lot of simple things not explained that make it appear too much of a hurdle for most people.
This is how I did it, and what worked for me. I strongly recommend buying test pieces and trying it, before relying on it in your circumstances.
One other thing, if you're looking for cheap sensors for things like temperature, pressure, wind direction and speed, luminence....then look at the 1-wire stuff available. I can't really afford a KNX based weather station, or luminence sensors, but 1-wire works out at about 1/10th of the price. I've bought the stuff, but I don't know if I can integrate it with LinuxMCE yet. Like I say, one step at a time, house first to get in by christmas, the LinuxMCE build will start first thing in the new year.....