Tommy,
Why are you changing out, have you ever used the pool. Filter wise it's basically upto what you like sand is easiest with backwash option, DE is next in line but will need DE every time you backwash and cartridge will need to be opened up to clean. A properly operated pool lends itself to twice a season not a biggy there. DE has best filtration then cartridge and the sand. Get largest filter you can get to go longer between cleanings.Heater with NG will serve you best in the shoulder season as heat pump is slow to heat and won't work in colder temps. Pump wise get largest hp for efficiency. SWCG work well as circupool has good reviews.