Are you kidding? Singing is not all about range. Brahms requiem is hard to sing - tessitura and long lines. Not just anyone can do it, or do it well.If the untrained person can't sing along, then you're writing in a way that relies on someone else's virtuosity to bring life and intensity to your music. I hate to be THAT guy, but I feel like singers are a dime a dozen? It doesn't cost a whole lot to drive to the nearest college/conservatory and offer pizza and some starbucks gift cards to 2-3 singers to sight read your work and nail it in 2-3 takes.
hiring a choir? pssshhh not in this life time, but a single person for an hour or so that doesn't even require equipment to perform the music should be an easy task. Infact plenty of elderly in my area sing for fun, you could visit a church or two on sunday and find a few who might be pushing the life expectancy up, but have a long and rich history of singing under their belt, and are more than interested in working with someone of a different generation. Even better, you might end up getting some really wild tales out of them too for free.
again, if it's in a typically singable range for an untrained person, anyone with any experience should be able to crush it with no real hitch, with the added bonus of being more memorable.
this goes up to like an F? a whole 5th below the infamous C. Still hits like a choo choo train. Infact, I would wager a random person off of the street would only struggle to hit the E and F, and most would be able to hover around the A and D without a hitch.
And yes, trained singers are a dime a dozen, and many a voice major would be happy to work on a project for little more than just being a part of something - assuming you don't approach someone going to the top conservatories. And certainly you can find trained older singers that don't sound old, but will have a wealth of musical experience.