making a gas pump

[M. Sage]

They're probably some flavor of polyethylene which is fine for holding gasoline, but check.

Modern fuel systems are often mostly plastic these days.

[Vash]

Don't forget about static. Its the main reason gasoline tanks are usually underground. Its ok to keep diesel above ground because it doesn't vaporize as easily and is therefore less likely to go up from one little spark.

[thorkyl]

HDPE plastics are what gas cans are made of
Look for this symbol on the drum
HDPE symbol #2.png

I use the blue drums your talking about for diesel at my place.
As far as I know they are HDPE

As for pumps
Transfuel Brand Fuel Transfer Pump 12v 20GPM W/Automatic Nozzle

Look for ones rated as explosion proof

For storage, if you add Sta-Bil to the drum then your fuel will last a year in a sealed drum with a pressure vent cap.
Equipment2U.com - Prevent Fuel Cap Kit & Adapter Kit #1910-5

Keep the drum under cover and do not fill it.
You can transport 600 pounds of gasoline including the drum without a hazmat license
50 gallons weighs about 315 pounds

I would not go more than 53 gallons in the drum, that way you have expansion room.
Also make sure you put a water trap filter on the pump so you trap any condensation that builds up on the inside of the drum from temp changes. Drive a ground rod into the ground ext to the drum(s) and strap it to the pump.

[M. Sage]

For what it's worth, regarding the weight of gasoline - 6 lbs/gallon is the quick calculation rule of thumb. 8lbs/gallon for water...

Why do I know this? No, seriously. Why do I know this?