The house was structurally sound, but the mechanicals were very dated and the property needed a minor facelift ($7K).

Structurally and mechanically the house was fine with the exception of the water heater.  

Take the cost of the home long term add the mortgage, taxes and true repair costs such as upkeep and new roofs , heating and cooling mechanicals etc over the years.

While I was/am not an inspector, I have good mechanical skills and construction fundamentals, so the walk thru was sufficient. 

Regarding "tax sale flipping," I think the mechanics of what you are describing would work, but I don't understand why an investor would buy the tax deed from you at a markup instead of buying it directly at the auction.  

As an investor, I take care with my flips to have all the majors covered (foundation/roof/mechanicals).

More often than not mechanics break down as a result of the house being foreclosed and not utilized for an extended period of time. 

Hey Amy, check out MAG Mechanical, and talk to Tom.  

Should I worry about a potential mechanical Lein on the property?

I assume major mechanical  updates.