Orbiter is pretty easy to set up. I always download the zip file, then extract it and move the folder where i want it to be. If you make sure the directory structure is preserved, which is usually the default, then Orbiter is already ready to go.
Installing Addons is just as easy. Most of them come in a zip file which you just need to extract into your orbiter folder and you are done.
Now the base installation in Orbiter is fine to take your first steps, but it lacks some nice to have features like sound and some higher quality graphics.
The textures are classified in levels. The higher the level, the higher the quality. Unfortunatley the main download site for the highest resolution textures (Level 11+) is currently down and the only way to get them is via Torrent. You can find the torrent links here: http://www.orbiter-forum.com/download.php
For Level 10 Earth and Mars textures you can go here: http://orbitersimulator.com/landsat-downloads/
In regards to Addons i would like to recommend that you download the following as they really enhance the experience:
OrbiterSound 3.5 and Universal Mmu 2.0 http://orbiter.dansteph.com/index.php?disp=d
OrbiterSound as you might imagine brings the simulation to life with sound. It's pretty easy to install. Just click the exe and point it to your Orbiter directory. Universial Mmu enhances Orbiter with the ability to add Astronauts and do spacewalks with them. Very neat addon to have.
I always recommend TransX as well, as it's the most powerful tool to get around in the Solarsystem. It is a bit hard to understand at first, but there are some great video tutorials on youtube. You might skip it though if you just want to focus on Apollo first. There are a lot of TransX version out there. Orbiter even has a default version. Most people recommend using the 3.13 version which you can find here: http://orbiter.quorg.org/releases/
I guess you have to check which version gives you the least problems for your tasks.
You might also try the DirectX 9 client as it enhances the graphics and in some cases even improves the framerate a lot. It does crash with some Addons, which is the reason i have never used it, but if you feel a bit experimental you can give it a try. Here is the link: http://koti.mbnet.fi/jarmonik/D3D9Client/
I strongly suggest that you read up on the possible problems with the client first before installing.
Now to the Apollo stuff. I have never installed them, so i have no insight into which is best. In regards to installing Addons it is usually just unpacking the zip or rar file into the orbiter directory. If it has an installer it probably will do all the work for you anyway. Could you link the two addons?
If you have any additional questions just go ahead an ask. Welcome to the Orbiter world!