Without good pics, your replies will be few. If you cannot do the work yourself, the monies spent for outsourced labor are seldom recovered.
As GTD wrote, without a photos, a more well-defined scope of work, and the answer to who
will restore the car, we can't give you a budget. We can tell you that it will come out to more than you plan at the outset.
Yes, I know the photos are coming.
What do you mean by "restore"? Just getting it running and back on the road, or ready to win first prize in a concours? The $ difference between these is significant. And will you do the work yourself, or have a professional do it? The answer to that question will have a big impact on the budget too.
I understand that the paint and body are OK and "all it really needs is the interior and mechanical stuff". But "mechanical stuff" isn't a very precise scope of work. I predict that once you get into it, you will find that more areas need attention than you had first thought. For example, any car that has sat for many years is going to need all of its rubber parts replaced - these decay with time, as well as mileage. At a minimum you should plan on replacing the giubo and going through the braking and fuel systems.
If you want to get a start on a budget, go to the websites of some of the Alfa parts suppliers (Centerline and Re-Originals here in the US, Classic Alfa and Alfaholics in the UK) and start noting the prices of carpet kits, door panels, upholstery, etc.