Old Timer needs help/advice

Hello guys, I'm just getting back into flight simulators. I go way back to Falcon 3.0 which ran in DOS. I have several basic questions which I'm sure have been answered somewhere, but answers could save me a lot of time experimenting. I currently have discs for FS2004 (which so far I have no had success getting to run on Windows 7 even using the patches here), FSX Gold Edition, and FSX Steam, FSW, and DCS 1.5.

1. I recently got FSX Gold. I uninstalled FSX Steam and then installed FSX 10 (Gold). I had to create my own shortcut even though I requested that one be created. It seems that all this does is reactivate the Steam edition.
Question: Is there any reason I can't have 3 versions of FSX installed at once - FS2004, FSX Gold, and FSX Steam.

2. What are the cons of having just FSX Steam? Do new aircraft still work with Steam version? Is Steam version equal or better to stand alone version?

3. What are the pros and cons of Lockheed's (I retired from there) PrePar3D?

4. I have a Ryzen 5 running at 4GHz with a GTX 1060 and 16gigs of 2400 memory. I think I can wait to upgrade video card to 1080 right? In other words, wait until I see I need it?

5. Should I invest in XPlane 11 to check it out? ( I heard you can't set up ILS approaches in XPlane)

6. Where is best place to get build drawings, specs, etc on contructing my own 'in home' cockpit?

Thanks in advance for any advice and answers. Steve