Should I request a receipt for this charge, even though it's below $600 and may not require a 1099 form?

It is easier to use as LLCs have requirements like separation of assets (personal and LLC assets are to be separate) in order to not be pierced for asset protection. 

I have a property ideal for conversion into a triplex with minimal renovation required.

If rehabbed did you check to see if they pulled required building permits ? 

If a partnership turns into a single member LLC due to a partner leaving, a new EIN is required but how would a business still keep the branding of their original name if creating an EIN requires a different name?

There shouldn't be any need to negotiate them as you would negotiate rent, they just are what they are, and the owner is required to provide proof of those costs to you.

Obviously they require electrical near by.

Find out what their requirements are.

I could put the 3.5% FHA loan route but not sure if I want to deal with FHA requirements.  

Bring on a partner with at least a decent FICO, experience in rehabbing would also help, but not required. 3.