I will be inheriting a tenant who is a retired custom car builder/mechanic and his wife.  Their unit is well maintained, and they currently do cleaning and yard maintenance on the side for other properties.  They have...

Even a smaller deck, if built by Tenants and their helpers, might not come out very well.  

I found a handyman on craigslist that said they could do it in 3-4 days and it would cost for him and his helper about 350 a day.

Unless it is just demo (and even that can be screwed up by a non experienced helper), too easy to have this take longer or get messed up and have to bring in a pro to fix it which would cost more than twice as much and take twice as long.

Due to covid they have no helpers and too much work.  

Is it unavoidable to become a business to pay for helpers in this scenario?

In Cedar Rapids, IA there is a non profit group of retired guys mostly with some volunteer day helpers that build ramps for free so maybe there is in your area too. 

The authors, David Osborn and Aaron Amuchastegui, suggested hiring a helper to investigate the prospects.

Hi Jacklyn,  I believe the generic advice of "get a side gig" to earn your down payment money means (today);  drive for uber, door dash, find a few local elderly who don't want to go shopping and be their helper/shopper. 

This was a large apartment, so it took approximately 3 months to complete working alone and sometimes with a helper (hired help is virtually nonexistent currently in NY).