I know very little about Marx trains. Marx had some great accessories though that play well Lionel size wise. Quality and detail wise, American Flyer was no slouch. I had never even seen Flyer trains until 10-12 years ago. About that time I added a basic vintage AF set that was just stupid cheap.
I really want to setup a layout in my basement incorporating all the O & O-27 stuff, the S gauge stuff, and a massive gauge slot car set that I have been sitting on for some years too.
American Flyer was great stuff, probably closer to scale model railroading than Lionel, but Lionel was just so popular that AF just never got much traction.
That's going to be some layout. Using forced perspective you can make it all work togethor nicely but will require a lot of space.
My O gauge equipment is all Lionel: O, O-27, Super O, & O-72. It's all in storage but once upon a time there was a 162 sq. ft., 2 level layout where it all worked togethor. I have an upcoming opportunity to run anything I want on a scale layout with 3 rail Gargraves track. I'm in the process of choosing an engine(s) to clean up and test so they are ready to go.
Lou