A flat world can be a very good way to get started. If you want to make it look more realistic then why not use the 'overworld' preset? I find it much easier to build a mountain than to get rid of one and try to make it look natural. Another advantage is that a flat world will have less things going on than a regular one, which means it'll be less taxing on your computer. No dark caves where mobs can spawn all the time, nor a massive area which needs be be constantly loaded / unloaded.
So that would be my suggestion: start with a superflat world (overworld preset) and just add everything you need.