If you decide to go for the smaller steel tank and it has rust in it, its an easy fix. remove the screen at the filler neck and insert a steel round bar, but dont let the bar touch the sides. Attach your positive DC (cant do this with ac)welding lead to the round bar. Take your ground lead and hook to the outside of the tank. Fill tank with water add a few spoonfuls of baking soda. Turn the amps of your welder down to 20amps and turn it on. It wont take but a few minutes and you will be able to see the water solution start bubbleing as the rust is pulled from the inside of the tank and deposited to your round rod at the filler neck. You can do this with a battery charger at lower amps, but more power the faster it works. I did this once to remove rust from inside a 1000gal hydroseeder tank. Left the welder running overnite, next day dumped the water and the inside of the tank looked almost like new metal. It might take a couple of trys to get the tank rust free, but once rust free, buy a can of gyptal paint and pour inside the tank and roll it around until you know the entire inside is coated, Drain out any excess paint, Let cure a couple of days before filling with oil. You tank will be rust free and the gyptal will keep it that way.