A couple years ago I inherited a 1980 Fiat 2000 from my pops. It has been sitting for ~12 years! She supposedly ran great when parked at a friends farm-garage for repairs to the rusted out door bottoms.
I finally have my own place and want to restore this vehicle!
Mechanically speaking, where do I begin this restoration adventure? I assume drain and refill all fluids.
Assuming that the engine and transmission do not need to be replaced, does anyone have a ballpark idea of what it would cost to restore her to original running condition? Definitely will need to repair/replace doors, a paint job, new top, and really not sure about under the hood from sitting so long...
Any suggestions or ideas are welcome.

I'd forget about the cosmetics for now. How was the car stored? Indoors on a concrete floor, or outside under a tarp? Don't assume that the structural components often discussed here are free of fatal rust. Examine carefully before spending any money on restoration.

Ensure that the engine and transmission are really functional. "Ran When Parked" is right up there with "I'll respect you in the morning" in famous sayings. After allowing a tablespoon of ATF to soak each cylinder overnite, try to turn the engine over with the plugs removed and fuel pump disconnected. If you don't hear any crashing noises:

Remove and drain fuel tank. Replace every rubber fuel line; don't forget filler elbow. Brake calipers are probably frozen. Easy to remove, since you'll be cutting the rubber hoses off and replacing them anyway. Don't forget the 5th hose near the diff. Rotors can't be turned, so budget for them, too.

Essentially you'll be replacing all moving rubber parts; timing and water pump belt and heater hoses, etc. Despite outward appearances, tires must be replaced after ten years, regardless of circumstances.

If the coolant was in decent condition when parked, you may be lucky with radiator/heater core leaks. A simple pressure test will reveal problems. If all that needs replacing are rubber parts, battery, tires, fluids and filters and a complete brake job, you should be running around for less than $1,000 if you do the work yourself. But be smart, and do things in order: structural rust check, engine turns by hand, engine runs, etc. etc.