They're lucky to have such a nice home with you.
If you want to get the adult cats neutered but cost is an issue, the RSPCA and Cats Protection have a farm cat neutering scheme where they trap (if necessary) the cats, neuter them and return them to the farm. When we had a branch of CP here it was one of the things I use to deal with and it didn't cost the farmer a penny. The biggest problem we use to have was farmers who were stuck in their ways and didn't believe that neutered cats would be good mousers which, of course is nonsense as they make better mousers if they're not always pregnant or out tomming.