For the short term having no money to produce a big budget epic is a good thing. If you had an unlimited budget to work with and access to all the actors and locations on this world and how ever many others that could be computer generated then you would not have to learn the basics and you could happily go off and produce your version of Dune.
If that is your goal then you do not need any help or advice, go forth and create your epic piece of what ever Dune was. Do not get me wrong Dune is an entertaining film, but it is in that accidental kind of way. Sort of like a Three Stooges kind of car wreck. The sad thing is that we can not say that for 99 percent of the mega budget flops that hit theaters each year. These movies are usually written and produced by those who never bothered to learn the basics.
Are you familiar with the films Batman the Dark Knight and Inception?
Christopher Nolan did both films. Wrote, produced and directed. This is a man who learned the basics while making low to no budget films.
Most people think that Nolan’s first film was Momento. It is a film with a million dollar budget and that was not where this great writer/filmmaker began. He started with a movie titled Following. It was a 6,000 dollar little movie that he wrote and shot on weekends for almost a year. It is the hard work and lessons learned while writing this screenplay is what helped to make him someone capable of creating a film as unique and massive as Inception over a decade later.
When you have to be inventive to solve problems rather than throwing money at the problem you will actually be learning the fundamentals that will help you years down the road to create epics.
Do not look at having to limit your locations, the scope and number of characters as a handicap. These are the lessons that will make you a better writer.
Keep in the back of your mind while doing the first draft the number of locations that you are using and the population of your screenplay. Remind yourself that locations cost money and that actors cost money. What about extras they work for free? Hey in the ultra low budget world your stars will most likely work for little to no money, but they have to be fed. Extras have to eat. Even if it is pizza and pop tarts they have to be fed. Food cost money, so extras cost money.
If you have a script that requires a cast of less than ten people then that cast can most likely be held together by the producer and or director for many weeks or months if the weekend way of shooting is required.
Keep these things in mind and try to write something every day. Even if you are just writing notes.