There aren't lower requirements for a drive in theater vs. any other movie theater. You need a fully conditioned projection booth, a screen of a specific size, then a projector designed to properly illuminate a screen of that size which can handle the movies which will be shown.

I would be hard pressed to tell you the distribution rights you will have or the movies you can show and how they will be delivered. I would be looking at Christie Digital or Sony for some information on their commercial cinema products. These typically run $80,000+ for the projectors for this.

Projectors are electronics, so you can't have them sitting outside, or even just sitting 'inside'. The space they sit in must be air, water, and critter proof. They must be cool in the Summer and warm in the Winter. So, the projection booth must be conditioned.