Lots of mythology already stated but behind that are some truths, too.
Size......well that all depends on what your objective is and you can make that as large or as small as you want it to be - nobody makes you build an empire that you spend a life time building and another lift time maintaining. My layout is in an 11'x16' room. Finished as well; never finished on maintaining.
Costs....again that all depends on what your objective is and you choose - nobody makes you go out and run massive steam or even large diesels, or large heavy electrics. Lots of smaller steam and diesels out there; just what is the basis for the demand for massive steam or even large diesels; compensation for inadequacies........
Track.....there's pre-fab track in 2 rail and/or you lay your own. I have no idea what sort of tolerances are needed to keep stuff from de-railing in 2-rail, but I must have met them....somehow.....oh wait, there's a track gauge. Dip in the track.....if you lay track that poorly using an uneven roadbed, you deserve derailments....that failure's on the human part of the partnership.
DC......terribly, horribly..........easy to deal with........and despite that I have no idea what I'm doing yet it all works, and everything you might need is readily available.....even for the dreaded horror of reversing loops is simple, basic DC. Almost 100 years of 2-rail running on DC and strangely enough, folks have figured out things
Availability......again that all depends on what your objective is and you choose - yup, there are gaps in era availability that exist independent of the number of rails. Yes, there is tons of stuff on secondary market; estates get dumped out for sale all the time and going to the better shows and meets one finds all sorts of fun, interesting stuff. Primary market is sadly shaky.....
I guess if your eyesight is good and your manual dexterity excellent, you should look at N or HO so you have a massive empire to spend a life time building and another lift time maintaining, while running massive steam or even large diesels, or large heavy electrics with a huge availability factor..........but wait, somehow the modelers of what, 75-80% of model railroading manage to keep their trains on the track running using DC. Will wonders never cease...
