[e46er]

start with RODI water and save headaches
make sure your sump has enough room for your skimmer footprint and a little bit extra for a safety net
a 20 G tank will not have enough room for a skimmer section/fuge section/ return section
you will want to make the return section as large as possible as this is where you will see the evaporation occur not as important if you use an auto top off

external pumps are more reliable in my experince but more $$
get a big enough return pump to run reactors for carbon, GFO, Pellets so much easier than individual powerheads running them

and for flow it really depends on what types of corals you want to keep and your rock placement and powerhead placement
i have around 8000 gph in my 120G tank