I like all of the above.
Since my first choice, On3 DSP&P is pretty much impossible to find even in kit form, I don't generally have the option of buying new or used...but rather stryene. I have built some Grandt Line kits...which I love...and I have an old Don Winter DSP&P way car kit.
I plan on building stuff that isn't comercially available...NKP 4-6-2s. 4-6-0s, passenger cars, and everything DSP&P. I don't plan on scratchbuilding any NKP hudsons, berks, or mikes but rather purchasing brass and plastic...I still need 7 more NKP hudsons ;-) I'll probably scratch build mason bogies since the components are available and the brass engines are just too expensive by comparison.
I guess my rule is, since I have very specific goals, is to buy what I can and build what I can't. The building is the most fun part for me, not operating, but I would like to have sectional model railroads of each road with the appropriate motive power, passenger cars, and freight cars.