I answered your similar question already. Please see that response.
Basically, if it's a lease-option, follow the procedure outlined in the option.
If it's a straight rental, have a Realtor do a CMA on the property.
It's not necessary for you to pay for an appraisal before going through the mortgage process.
From your other question, it's clear you're talking about an appraisal or some sort of valuation. As I said, for that, get a Realtor to do a CMA (unless an option specifies some other process). That won't cost you any money.
But in this question you're just asking about an "evaluation." One thing you should do--and I didn't mention this in my other answer--is get a home inspection. That'll do two things. First, it'll give you a good idea of any problems that you may not be aware of. HVAC, roof, etc. Second, it'll give you additional negotiating leverage with the owners.
Example (from my other answer): The owners want $300,000. You think it's only worth $275,000...or the CMA comes in at $275,000. You have the home inspection and the inspector finds some problems. (Tip: They ALWAYS do. That's what they're paid for.) So the inspector identifies $6,000 in actual or potential problems--maybe an old HVAC unit, or an old hot water heater, or some other issues. You then present the CMA plus the home inspection to the owners. "The CMA found that your property is only worth $275,000. And the home inspection says it needs $6,000 in repairs. So our maximum offer has to be $269,000."
Hope that helps.