For a 300 gallon system, I'd go with am 8' snap set kiddie pool, a submersible pond pump, and my bead/scrubbie filter. $50 for the filter, $50 for the pool base, $60 for the pump, $15 for the pool, and another $100 for plumbing. You can add a waterbed heater for about $15 off ebay to keep it from freezing.

Density depends on the fish and how big they are. I only deal with tilapia. You could probably do 50 tilapia in a system like that.