You will be sacrificing getting on with your real estate experience and holdings, but would be doing a good thing for aging parents and will have a good asset paid down and ready when you are both ready for other living arrangements.  

Mario, while it can be difficult to find someone interested in the kind of arrangement you describe, you also may not be legally entitled to any rent for a residential unit that does not have its own certificate of occupancy.  

That contract arrangement would be void.  

This has been their arrangement for almost twenty years (the tenant has lived in three of the landlord's properties, all located in California) and from inception there was never a written agreement for this arrangement.Now the landlord wants to have them sign a lease and write them checks for the work that is being performed, which apparently is to be cashed and returned right back to the landlord as the rent payment.

I arranged a mover for them and the day I provided them with the security deposit (it was actually given directly to their new landlord) they were gone and the problem was solved in less than 4 days.

Your agent can also make arrangements to show the property and write your offer.  

Were you in contact with other real estate investors beforehand and worked out arrangements to workout a double close?

Can a tenants in common agreement include an arrangement that would allow me to buy "shares" of ownership over time?

It protects them and allows you to keep your holding costs down a little.I've never done it, but I've arranged financing for it a couple times and had the deal fall through (knock on wood).Beset of luck!

A deal was struck up soon after the initial contact, but the time difference added more time to getting the contracts arranged properly, sending and receiving, to the final signature.