I agree with most of the above estimates. The car may be worth $5K, but only if you are taking on a labor of love. If you think you can restore the car and sell it for a profit - forget it.
Make sure the car is a Royale and not a regular REO.
Don't bother with the engine and transmission - you'll have to rebuild them. It will cost you less money to get a rebuilt Chevy 350 and a 700r4 transmission.
Test fit the fenders, brake master cylinder and steering linkage. You may have to do substantial refitting.
You're going to have to rebuild the woodwork yourself. It will be prohibitively expensive to have someone else to that for you. There are plenty of people on this forum who will give you advice.
Start categorizing what parts are there and what are missing. Especially door handles, window mechanisms, side mounts, bumpers, luggage rack, seats, etc. Too many missing parts would make this project too costly.
One big question: why didn't the owner restore the car? What was preventing him from finishing? If it was lack of parts or money, why do you think you can finish the car?
I love REOs and would like to see this one restored to its original appearance, even though it has a modern drive train. It will be a beautiful car. But make sure you jump into this project with eyes wide open. It's going to cost you $$$. Frankly, I'd offer $3K and promise to finish the car.