I'm not an expert on these cars but since nobody else responded, here are some hints:
Angela Cherrett was mentioned in this thread. I hope this answers the question about experts.
With respect to a restorer in Europe, Fabio Caligaris in Italy may be your man. I've seen a 6C 1750 in his excellent restoration shop near Milan when I visited him 3 years ago. He is the technical advisor for the 6c2500 register (see http://www.6c2500.org for contact information) and is a very nice guy who speaks English.
I got a kick out of the link to Auto Restorations in Christchurch. http://autorestorations.co.nz/our-work/Classic-cars/restored/1930-Alfa-Romeo-1750GS.htm ("I know that car from somewhere....") Anyway, they are Fantastic, but check your exchange rate. We've had another car done by Bruce and Gavin's Company many years ago, a 1947 Delahaye 175 "one-off". Both won Best in Class at Pebble Beach Concours and the Delahaye also won the French Trophy. In addition long-time friends have done at least six cars (Aston Martin DB3S, Ferrari 196S Dino Sport, Maserati 300S, Ferrari 375 Plus, Alfa Tipo C, Baldwin Mercury Special) if I recall correctly. Each won at Pebble Beach and also raced at Laguna Seca. This August one of the Auto Restorations Guy's was visiting during Monterey Historic's / Pebble Beach Concours Week and said "Hello". He really liked seeing the Alfa again after many years and was pleased to see it run so well at the track. They are top notch.
Considering the significant advantage due to exchange rates, I would also strongly suggest Phil Reilly & Company http://www.philreillyco.com/ for the mechanical restoration with all body work done by Steve Moal Coachbuilders http://www.moal.com/index.html. They have done at least two cars for us, including the Alfa following a major shunt. They are well known among vintage racers and collectors, are very private, costly and extremely selective about their work.