Yes, there are many free add-ons available for Microsoft Flight Simulator X.
On the website of the flight simulator ( Microsoft ) , their is an free demo with 4 airplanes and a part of the world where you can fly in ( carraibes ) . If you want the full version, you need to pay for it, but if you like , you can add more planes into your demo by flight simulator addons ( just type in google "fsx addons")
You can't, you have to buy it.
You cannot download Microsoft Flight Simulator 2004, you have to buy it.
Yes, multiplayer on Microsoft Flight Simulator X is free.
You can play Microsoft Flight Simulator 2004 multiplayer free in the link provided below this question.
There is one official add-on for Microsoft Flight Simulator X called Microsoft Flight Simulator X: Acceleration. There are hundreds if not thousands of unofficial plane add-ons available from many sites.
depending on what you want . Down load, add to the relevant effect folder , remember that there are many effects folders for differing areas of the simulator to process. ALWAYS backup your original files before launching addons
I have found a download for the Embraer 135 for Microsoft Flight Simulator X. I have put the link to the download below this question. To download you must register with the website but registration is free.
there are many sites that have free downloads for many of the aircraft types
There are a number of free downloadable models available across the internet with good quality "how to " files attached.
Some websites charge you for add-on aircraft while others are free.
one possible option is that you go in to Google and type 'free Microsoft ww2 flight simulator'. you can then download the most recent version. there should be one at the bottom on the 1st page that has the URL www.download.com/windows/flight-simulator-games/1921-7544_4-0.HTML. please note this may only be a trial version.