Improvements of my rolling stock

[doctorwayne]

....In addition, on an operating layout, there are so many other tasks -- decoders, scenery, wiring, track maintenance, operation, etc etc that you have to budget the relative amounts of time and money you spend on particular aspects....

Jwb makes some good points. Unless you enjoy upgrading low cost (or even expensive) rolling stock with better details, as do I, much of what's available requires little to make it useable. My must-dos are Kadee couplers and perhaps some weathering, and I don't even bother to change wheels unless the original ones are faulty.
Beyond that, A-Line (or scratchbuilt) stirrup steps give a good visual return for not too much time or money, but most other "upgrades" are primarily for your own self-satisfaction. And, be warned: one foray into adding free-standing grabirons or more detailed brake gear often leads to a "need" to bring all the other cars up to the same standard. It can be enjoyable work, but it also eats time voraciously.

Wayne