For a 1980SC Targa non matching numbers car with a deteriorated second paint job and lack of original Fuchs at a price tag of $20k I would not say that it is a steal. But add $25K in parts/paint and a lot of time with sweat equity the car can be a nice $45K car.
I am the third owner of my 1980SC Targa and I payed $12.5K for my car in 1991 and since then I have invested $10K in an engine rebuild in 1998, $25K in parts/paint since 2002. The bulk of the restoration work engine, paint, Targa top, suspension, interior was done by myself in my garage and I continue to do all my own maintenance. Today, I estimate my car is worth about as much as I have money invested in it, $48K to a potential buyer. Fortunately, It was never about the money spent it was more about the journey over the past 32 years of ownership. I have no desire to part with the car because every time I drive it I get a poop eating grin on my face and that is priceless.
You have to ask yourself “ how much do you really want to own an air cooled Porsche 911SC Targa” and how hard are you willing to work to get it.