Unless you are a really good wrench, send the trans out to be refurbished. The 912 motor will be just as expensive to redo as finding a Proper and Correct powerplant. Find an engine, sell the 912 one and on't look back. P&C...you'll learn to hate that phrase.
AS to where to start, cleaning the rust and determine whether that panel needs to be relpaced. POR 15 it, cut out, patch grind....you have enuff there for a while. Pull the tank see what shape it is, pull all the gas lines, brake lines, wheel cylinders....get back to me when done.....
Sign up on the 356 registry. Lot's of help over there. They can be a bit a bunch elitist buggers but they have a lot of insight.
NEVER if you can help it buy anything from Sierra Madre Products.
Stoddard's, NLA and a few others have nice reproduction pieces. Don't anything NOS unless you have an open wallet. One of the biggest expenses I had was a
steering wheel.....use a cheapo until you know what you really want.
Don't sell anything untill you have to. Chrome VW wheels fit and you can have them made to order at Jbugs. Don't buy hubcaps until it's ready for the road.....if at all.
Any other questions....ask. Good Luck.