As to what buildings you make, it is fun to have a theme or idea of how they all inter-relate. It can be anything, such as a theme of making a '50s small town, or silly - take the Blazing Saddles approach (every business in town was named "Johnson"). Some theme, serious or silly , makes it more, well, fun. I have one area of my layout that is nothing but car dealers (so I have places to display diecast cars) and another where I try to reproduce the main street in my grandfather's hometown when I was a kid. Regardless, some theme you like makes it more fun . . .
Your idea of buildings that you can " lift out and replace" is a great one and I recommend taking that route. The reasons are: First, you can work on the building off the layout. Second, you can move buildings around: about 30% of the buildings on my layout have moved as I gradually found a better arrangement for them. Third, you can build more buildings than will fit, switching them out from time to time: I'm at this point now: when I complete new building it often has to replace an old one because there is no room for all of them.
Anyway, there are two ways to do this "lift off and replace." The first is the obvious approach - build the building itself freestanding but never glue it to the layout, as in the two pictures below. However, you might learn some things, as I did, by studying the Woodland Scenics built-ups - that company very cleverly glues 'attachments' to their edges of buildings like trashcans and dumsters against the back and side wall, a dog sleeping near the front door, bikes or scooters leaning against a side wall, etc., that add a lot to the building's look when on the layout. Completes the look once the building is in place. I do a lot of this now, having learned from them. Anyway, an example of such a building on my layout -
The Indian Restaurant is there . . .

Two seconds later, the Indian Restaurant is not there . . . (I put it back after this photo)

The second approach, which I use more often, is to build the building and area immediately around it as a "vignette" on a small piece of wood. I then leave a hole or a space sized for that piece on the layout. I build small vignettes on 1/8 inch model plywood, larger ones on 1/4" plywood or foamboard if light. The pallet below is a modified/bashed Woodland Scenics Harley shop turned into a moonshiner's garage. I did the entire 14" x 11" scene off the layout and then transfer it to "hole" in the layout where it fits, doing all the tiny detail and wiring it (there is a welder under the '50 Ford as a guy under there puts on a tiny set of headers) before just transferring it to the layout. If this approach interests you, I posted pictures earlier this past week on the 3-rail forum about my "Race Team Headquarters" which is the largest pallet I have made so far that show more detail on the method.
Lucus Doolin's Moonshiners garage is a 14" x 11" assembly, complete, that slips into a 14" x 11" hole on the layout.
