I think the most important decision to make is where you want to go in the hobby. Do you want to build a model RR or do you want to build a collection. If you want to build a model RR, you may want to seriously consider sticking with scale stuff. As you study the history and look at more pictures of prototypes, the non-scale stuff will probably begin to disappoint you. On the other hand if you like gathering the various things the are nostalgic to you, then you can focus on those types of things regardless of traditional or scale. Part of this decision might be to consider do you like to build stuff or just buy it off the shelf and put it on the track. How much effort beyond purchasing do you want to put into the hobby.
If you go scale, the second decision will be what to do about a layout. If you have less space, then it is worth considering smaller prototypes. Most RRs regardless of where and when had a lot of smaller stuff in addtion to the big glamorous 4-12-2 or big boys to T-1s on the PRR. Learning about your prototypes will be another aspect of the hobby that is fun for many people. The PRR had over 3000 consolidations of various classes and about 50 T1s. Another thing about going scale, it sorta tends to lead you to be more selective and choose one or two prototypes and try to obtain models of their equipment. and as you learn more about these prototypes, then the focus of your purchases can become narrower, because if your selected prototype did not have an XYZ boxcar, there is no need to buy one, no matter how nice it looks. This requires learning some restraint (spoken from experience) but does reduce the amount of spending, or more precisely focuses it. This applies to steamers and diesels and cabooses and freight and passenger cars.
Of course, another course is just to buy what you like, but then what is the definition of what you like?
