Mine was a 320g. I used a Mak4 for a return pump and the sump was 65gallons.
So 55g would probably work. Pump around 1200gph after head calculation is about as low as I would go. This doesn't include in-tank circulation such as powerheads etc.
Skimmer, for a 300g isn't going to be cheap now matter what you do if you want an effective skimmer that isn't undersized. You really want a recirculating
needle wheel type skimmer preferable with the deltec needle-wheel. Reefmania version I would probably do. Even a DIY the $$ of the pumps alone will set you back as you probably need 3-4 of them at $90 each.
The most important question is how old is your house? Will it be able to handle the humidity a tank that size will cause? Do you have a fresh-air exchanger? This was the ultimate demise of my setup and had to tear it down at the 2 years mark.