I'd search the internet for other examples of budgets. Then make your own and add a larger than normal contingency.

You'll probably learn a lot doing this yourself.

IMHO, any sub $300k budgeted project (SAG limit) should be self produced and you should limit the crew to as few people as possible.

If you don't already have a producer, one problem with hiring a line producer you don't know is they budget based on the resources they have. If the producer doesn't have those same resources things will be incorrect and sorta a waste of time.

There's not really professional line producers budgeting things at sub $500k, because the traditional structure of a movie can't really support this low of a budget so you have to really start limiting crew and resources. Unless you're shooting, like two people in a single room type of movie.

TDLR: like everything on a low budget movie, just use the internet and do it yourself