OK, you're going to need a few things.....
1) A projector. You sound like you a have a budget for that.
2) A screen...You CAN DIY that.
3) Something to play disks... DVD / BluRay player.
4) Speakers....Something to hear the movie.
5) An amplifier/ receiver to send the sound to the speakers.
Have you thought aobut these things? Do you have a total budget in mind?
For the porjector, just about any recent (3-4 year old model) projector will work for you. Personally, I use an old business class projector from 7 or 8 years ago. They tend to be really bright and also tend to be smaller and more compact so they can work well.
You will find that to start, ANY flat surface will work, even the side of the house or a shed, etc.
You can knock out items 3,4 and 5 with a Home Theater in a Box (HTiB as it's known around here).
For my deck theater, I bought the cheapest, most powerful HTiB I could find for the money I wanted to spend. At the time, they didn't include the disk player the way many (if not most) of them do now, so I added a $29 DVD player. Connect up the speakers, spread them around the area, and you have full 5.1 surround sound. If you are only doing ot a couple of times a year, just place the speakers around your area as best you can.
If you find that you are doing it more often, then you can start to upgrade. Get outdoor speakers and speaker wire that you can leave outside (the low voltage wiring for garden lighting works great and is esay to find locally). You can build a frame for the screen that makes it easy to put up and take down. I found a small, plastic cabinet designed for garden tools with two shelves and after drilling a couple of holes, I leave my stuff in there all season. Then I just put up the screen and bring out and hook up the porjector and I'm ready to go in about 5 minutes.
From there, it's up to your imagination and budget.
I highly recommend looking at this site: www.backyardtheater,com
You'll find PLENTY of ideas and help there. And if you scroll down the front page far enough, you'll see my deck!