Hello Yvette,
Trainz TS12 is still a good product, however, it's quite old now. If you can I recommend looking at the latest, even skipping T:ANE, and go directly to TRS2019 which is the current product. The naysayers will say don't bother, but since you are new at this, go for the current version so let me explain because you are not attempting at fixing an existing route and hobbling something already being worked on, and are starting right from square one.
TRS2019 is based on the T:ANE code and has many features which did not exist in T:ANE. In some ways it's what T:ANE was supposed to be, but never quite got there due to the newness of the technology used in that product, time constraints on marketing, etc. There are many improvements internally making TRS2019 more responsive with better graphics even while using the same settings as those found in T:ANE.
Now this of course comes with the given that you will need a decent PC to run this. If you could, please do post here your machine specs and we can help you to ensure you don't purchase TRS2019 and not have it perform well.
Now regarding the free content. The quality is going to vary due to the age of the offerings with some content being better made than others. As far as quantity, there's well over 560K assets available for download from the Download Station in addition to the nearly 9,000 assets available built-in, meaning included with the program all ready for use on your own creations. This doesn't include content available from a countless number third-party websites, or partners such as RR-Mods and Jointed Rail.
This answer only scratches the surface. As a longtime Trainz users, someone who has used the product since December 2003, I will say that this is a very addicting program that will use up many, many hours in your day as you explore, build, and create a world all within the confines of your PC.
Hope this helps.