2007 Ford Mustang (100k Miles & Manual Transmission), owner says cylinders have low pressure and needs a new engine. It is listed at 2800$
Is this a worthwhile steal or a messy project
I wouldn't even think about that car.
Depends on your resources. How much is to rebuild that engine or get a re-man? What is the value of the car with a new engine installed? Is it worth it to you personally?
Do the maths. Find out what a new or rebuilt (from a trustworthy source) engine costs. Find out how much the labor is to install the engine. And find out how much a similar model in similar condition with working engine Mustang would cost.
If the engine + labor + cost > similar car, probably not a good idea. If it is significantly less, maybe a good idea.