Start with spending on power, ZW, ZW-L, etc. depending on how deep you plan to go with electronics, the ZW-L might be worth the extra loot. You never hear someone complain they bought too much power; just that they want more
I surely wouldn't go smaller than a KW on 15x20 . (maybe an LW for each loop)
Try SCARM (capable/3d) and Anyrail (simple) free versions if you have Windows.
You don't "need" a computer, but it is fun, doesn't wake everyone at 3am, can save time and effort, and are pretty accurate. It saves pencils too.
☢️Save the pencils!☢️😬🙃
Look at GarGraves track. It's worth it to most folk. Ross turnouts maybe. Harder wiring GG/Ross turnouts, but that's mostly so it is easier to customize; they work real well..... except on pre-war fat drivers like Marx. Prewar/tin thoughts mean a plain loop(no switches, no uncoupler tracks(need skinnier magnet/ custom) or using Marx 0-27 prewar turnouts or importing larger curve ETS track from Czk. (keep in mind while pricing GG/R/etc , not all turnouts include the sw. motors and linkage. Many motors also mount under the layout and take extra work over others)
Im not a FasTrack fan.
Do check out Miane benchwork kits, or at least mind their style. 2x4 overkill isn't as good as thinner wider boards cut and fit to specific functions.
Use a thick foam +3/4" and/or real Homasote as a benchtopper to cut down on noise. Tons of threads on sound deadening too. Lots of cross brace supports, foam & H-sote are the effective things you can start now.
(Trains are much louder on a table than on a floor. Carpeting/rugs will help tons for absorbing sound from the underside that the floor will deflect back up at you. A thick curtain along the front edge, down to the floor, also helps trap sound (and hides all your junk).
Menards track would be a lower cost alternative for new tubular rails.
New track is worth the price. Old works; sure. But on new track your stuff, old or new, will run better period.
Lift ups and drop downs are a PITA IMO. We did a wide one, then narrower, then gone. I'd avoid it unless it's a must for design; and you seem to have an open slate.
I'd use reverse loops or bent dogbones, turning around at the doors/steps/whatever. Islands at these ends can be wider than benchwork along the wall for wide curves on the rev.loops. Reverse loops are nice as you never have to touch a loco to turn it around. (this assumes you are looper, and not as interested in a more prototypical switching layout)
Another option might be to make each rev.loop a regular loop. "Two towns with 2 (?) mainline connection(s) between them".
The other post above has a geat point, How do you run your trains? Looper? Totally proto logistics, distances, etc? Switching? Knowing your habits and yeilding to them is a better way to build that some "dream" you'll never use as envisioned.
Maybe add another island on the 20' wall for a yard, another loop etc (or keep it in mind for a later extension)
Put the controls where you can overlook it all, on the outside edge facing in. Otherwise, the whiplash effect to look behind you at the train is a PITA, especially when you already have a little kink in the neck that day 😧