Inheriting underpaying, 11-year tenant

[Dillon Alder]

I am under contract on a SFH where there are tenants in place who have never missed a payment in 11 years. They are a retired couple in their late 60's; the previous landlord hasn't maintained the property perfectly or kept up with market rent. This property is in a small town where my parents know the tenants and my parents live within a block of the property. Current rent: $625. If I put $15,000-20,000 rehab I could get ~$850-$900 in rent. The rent as is could be in the $700's. It is a 3bd/1ba around 1200 sqft. (My dad is a plumber and could add another bathroom) $75,000 purchase. I am wondering what I should do. I am not planning on raising rent right away because they have been great tenants and they always pay. I don't think I could do any renovations while they are in the home. What options do I have? Thanks in advance