Hi, first of all, welcome to X-plane!
Feel free to use the website search engine to look for keywords, because many of your questions have already been answered on this forum.
I will still take the time to answer as many questioons as possible, even though some of the answers will just reflect my point of view and it's always possible to see things differently.
Using a flight simulator is always helpful for real life, but it can also make you take bad habits. It all depends on how you use it. As good as it can be, a simulator will never be able to reproduce everything single sensation you can get in real life. It can be seen as an asset (you can focus of specific aspects of the flight and have more mental ressource available to learn) but also as a drawback (the real experience will always be different and most of the time more complex and stressfull). It is a very complicated question, but overall, I would say that having an instructor involved in your simulation experience will always be a good thing.
It is not very clear.. is you budget for everything? Computer+screen+yoke+rudder peddals?!? Would you rather choose a laptop? Would you consider buying second hand equipement?
If I had to buy a computer for X-plane today, I would probably spend at least 1000$ in the PC tower only to have a GTX1070, I5 or I7 processor, 8 or 18Gb of RAM, 480Gb SSD for the sim.
You could consider a cheap second hand joystick to start with, if you are short on budget... so there are many many options and it all depends on your priorities.
These links might be helpful :
Steam is a only a reseller, so they do not know the product, it adds another layer when you want to solve a problem (and you will have some problems, especially if you want to tweak your simulator).
Updates are available later on Steam (sometimes several weeks)
You will directly support Laminar Research (I am not part of the team) if you buy the digital or DVD version from X-plane.com
You can look for Steam in the searchbox to read the different threads about it.
If you buy the licence from Laminar Research, 1 product key = 1 instance running at a time. That's the only restriction.
You can install the same things on Steam version and on "Laminar Research" version.
THE website for third party products : X-plane.org
Not everything is there, but almost.
Sometimes it's payware, sometimes it's free. In general, payware is of a good/better quality, but not always.
When it comes to aircraft, if you want to go deep into system simulation, you will almost always have to choose payware aircraft, except for the B737. The choice is now quite vast and things are improving. Please don't ask me for links, everything in on X-plane.org (search engines, forums...) and many interesting videos (tutoriels, top10...) are on YouTube too..
60$ is a normal price for a very good airliner and there are sales sometimes.
You never need to pay for an airport in X-plane. Not all airports are accurately modelled in X-plane default, but things are improving quite fast thanks to many volunteer artists. Everybody can create an airport with a free software provided by Laminar Research , WED.
Everything is centralised here :
And more visually here :
(updated every couple of weeks)
Every 6 months or so, Laminar Research publishes an update with all the new airport submissions included. Today, 1 airport out of 7, in the world, is very accurately modelled, with standardised 3D buildings in the default version of X-plane.
In addition to that, you can manually install freeware airports from X-plane.org, with customized 3D objects, or you can also buy some.
I think it's possible even though I have never tried.
But you can definitely fly on Vatsim or IVAO if you spend some time installing the appropriate plugins and addons :
Vatsim and IVAO are independent, but both connect pilots flying from different platforms (Flight simulator, P3D, X-plane)
Pilotedge is the equivalent, but payware (much more realistic) :
Many videos on YouTube...
Well, that's all :)