In pondering what to write about for my $1000 movie, I harken back to the days of low budget master Roger Corman. Here was a guy who could squeeze blood from a stone, reusing locations (and often footage) for different movies and was even known to start tearing pages from the script if the director was behind. He used only one take per shot, and was bet that he couldn't shoot a feature in two days--and he won! The film was the original Little Shop of Horrors (1960).
Obviously inspired by Corman, the micro-budget rule is to assess what you have access to and write your script around it. If you can get into a gothic house, shoot a horror movie. If you have cops in the family, make something with an arrest scene. This is what I call writing from the "outside in", where resources determine your story and screenplay.
I can see the wisdom in this (especially if you budget is extremely low or nonexistent), but feel that it's inherently flawed from the creative level. When you set your parameters on a story based on tangible things, you are immediately shackling yourself and limiting your imagination. You should be writing about what can be done, not what should be done.
What I prefer is to write whatever story I like, then adapt to get it shot. I had no idea where I was going to find a paved road near a wooded area when I wrote Middle of Nowhere. I just penned a story that inspired me, then found the location later. Had I thought "I don't have that location in my current quiver", I never would have written or made that film, which would have been very sad for me.
I say don't worry about anything when writing, just write and get your script done. Logic will dictate that you don't write a period piece, or something with epic battle scenes, but put the story in a contemporary setting and make the battle between two people and it's doable. It will test your problem solving ability as a director to get the script to screen, but that's the director's job. The writer's job is to write and write well.
There is always an answer to a problem posed by the script, and it doesn't have to involve spending money. Your creativity and fortitude will be tested as you come up with solutions to "impossible" problems, and you will grow and develop as a filmmaker. Aiming high and missing is always better than aiming low and hitting. You may even surprise yourself and hit that lofty target, which wasn't on a list of any kind.