There is just too much fluctuation to give answers to this.

For my investors, the currency exchange fluctuations affect their returns by as much as 2-3% in both directions.  

I prefer these more than other asset classes due to the physical nature and the fact that if you're not in a coastal market then price fluctuations tend to be stable.  

This used to be a staple of American economics for decades, but it is no longer the case, as too many other factors cause the bond to fluctuate now.

Or is it that market rents tend not to fluctuate drastically for any significant market change

There are large fluctuations in what each plan has to offer which obviously makes up for the pricing differences. 

I'm unsure if I can include that in her monthly income as I'm sure it fluctuates and isn't something I can truly verify because it's not part of her paycheck but a separate deposit every month.

My advice would be to start out in a market that does not have large value fluctuations tied to the economy.

There are many reasons a seller would prefer to finance it such as avoiding capital gains and having a guaranteed monthly check that will not fluctuate like to stock market does.

I'd say 5% would be very attractive, and 8% would feel a little high, so that's a realistic range, though it's tied somewhat to your financial strength as a borrower (this range will also fluctuate with market conditions, so if you're reading this post in the future: 1) Congratulations for surviving 2020!