If that is the case, I would allow your current tenant to extend and then use the extra time to find a new tenant. 

I also plan to extend a sun room or enclosed porch, which is to be connected to garage (to be built) along with mudroom.

I'm trying to determine if any statutory tolling would apply here to extend the SOL from 2015, when the judgement was made. 

Emails mean nothing, you need the Final Release form....you can google up FL contractor lien laws, Final Release.

Give the seller the down payment and structure a loan for a short period of time, like 1 year ( always try to negotiate the right to extend that loan upfront if possible) or more.

Trying to protect myself also against the seller extending the lease or changing the terms such as the monthly amount, etc.

I also plan to extend enclosed porch or sunroom in the backyard, and want to connect garage and this enclosed porch if possible.In this case, should I hire a general contractor or individual contractor for roof, garage, porch, etc?

Not sure your insurance carrier or lawyer would be happy, but it’s clearly an under-served marketMaybe have all tenants sign waivers that release liability.

Obtained quit-claims and in the process of getting Medicaid to release lien.

Yes, the GC emailed us a scanned, signed, notarized unconditional lien release for final payment.