This forum is a great place for free advice. Usually, the free advice is worth a lot more than the price, but occasionally you'll get advice you didn't want.
Nothing wrong with sticking with good ol' tubular track from the 1960s. Several manufacturers have come out with track that has roadbed molded onto it such as Lionel's FasTrack. It's nice, especially on carpet, but for my in-progress permanent layout, I stuck with O27 tubular.
In terms of rolling stock, postwar and modern vintage run nicely together. If you buy new rolling stock, be aware that with variety came a variety of sizes. You mentioned O and O27 which, today, are lumped together as Traditional. The modern era introduced 1:48 O SCALE which, in general, are more detailed, larger, and more expensive.
I'm a fan of dipping one's toe into the shallow end, so with that in mind, if you buy new engines and want to try out the modern electronics with a remote control, I'd recommend Lionel's Lionchief or Lionchief Plus. Nice thing about them is that they run on a conventional layout without having to interface any other electronics (computers, receivers, etc.). MTH's DCS System requires a Track Interface Unit, and Lionel's Train Master Command Control (TMCC) and Legacy systems require command bases. You don't have to be a rocket scientist, but they do add a layer of complexity that Lionchief avoids.
Hope that helps.