50s ranch,w/multiple owners/additions, major plumbing problems, needs remodeling/ repairs to family room, redecorating throughout. How do we determine whether to sell "as is" or spend thousands (supposing we can find reliable, competent contractors) improving and updating? Physically unable/unskilled to do much of the work ourselves (illnesses).

I might suggest that you find a trustworthy Realtor that has had some success in your neighborhood. Find out about your local market, disclosure responsibilities/ laws. Best to get this advice first. I rather doubt that you will find a reputable lender that is willing to write a mortgage on a property in the condition you describe.
Best Wishes.

Gary has a great idea to speak with a realtor. See what the market is like and if you can sell the house as is. Also what it would market for. There may be some small changes that can help the price without breaking the bank. (fresh paint some staging etc.) A really good realtor should be able to help you decide your course of action. They may also have contractors they can refer you too.

Best of luck.