After 25 years, the flooring, paint, appliances and even plumbing fixtures likely all need replacing.

So, you will now have 1 mortgage on the property to pay moving forward because all of your previous money owed was replaced by the new mortgage after refinancing. 

I would allow their lease to expire and replace them with new tenants if they are unwilling to pay the increased amount, which at an extra $300/month I highly doubt they would. 

As COVID has impacted office and retail leases there is a new tenant now to help replace these lost revenues.

Also, do you normally get a policy that gets you full replacement cost or actual cash value?

Chicken and egg - They don't think they can find a replacement they like.

.- A lot can go wrong when your replacing electric, plumbing, drywall, HVAC, framing.

After the window replacement, some cosmetic work, leveling, and rewiring the house will be livable and ready to rent.My question is after I finish establishing the new LLC and have the property transferred to it, how do I apply for a line of credit to afford all the work to be done?

If you have a boiler definitely don't change it is easier and cheaper to replace with another gas boiler then convert to electric. 

Tenant shall be charged for all repairs or replacements caused by Tenant, pets, guests or licensees of Tenant, excluding ordinary wear and tear.