I have a degree in mechanical engineering and ~5 years’ experience in the field.

I would charge the tenant for the door upon move out.When you purchase turnkey, you need to make sure they replace mechanicals.

Depending on the finishings and how much mechanical and engineering work the property may need, $150k may be accurate. 

The tenants have been long term, so I honestly wouldn't be surprised if the current landlord hasn't kept up with a lot of codes and regulations.You would need some of them such as a single rental license, and others would be a maybe depending on the mechanicals of the property. 

Take care of the mechanical parts of the home and write them off at tax time.

. - Land/zoning/UDO/flood maps/property drainage - Sewer - Foundation/structural - Measure the property yourself to make sure it is in the ball park of the stated sq'- Plumbing / Electrical / Mechanical - Roof- Cosmetic ... this is usually the only obvious thing and has the fewest expensive surprises. 

They had completed a lot of rehab & new mechanicals & appliances. 

If you have a basement with mechanicals in it that also factors in.

All mechanical systems need replacement. 

The two major ones I dont mind sharing and we use when an emergency arises but are more costly (however you pay for reliability around here) - are Blue Dot for mechanical and plumbing and Snake N Rooter for Plumbing (sewer/drain) - however Snake N Rooter's what I call cost to simply break out the tool box and show up (min charge) is about $35-50 or so more than Blue Dot's min charge.