Firstly...good choice.
I'll try answer some questions..

1) Is it common to fake them?
As far as I know there are no outright fakes made in some factory somewhere in asia, but incorrect models are often put together from spare parts from other models or watches.

2) What are the telltale signs?
Things like wrong hands and refinished dials, or wrong case, wrong movement, wrong bezel. You can study the pictures in the Chronograph galleries on this OntheDash site to be sure the watch is correct.

3) What does CAL 11 and CAL 12 mean?
They are the movement references; Calibre 11 is the earlier one, replaced by the Calibre 12. There is some slight difference I can't remember, better experts can explain (search the forum on the topic, I think there was an explanation a month or two ago).

3) Is the Viceroy a cheaper. different model to the Siffert?
The viceroy is the most common version of the Autavia. Both the viceroy and the siffert are different autavia models. The viceroy was offered in a cigarette promotion, I think thats why it was common. It's common (so much cheaper) but it is also classic.
I've attached a photo of the viceroy and siffert so you can see the differences.

4) It is an autavia chronometer with blue hands does that mean it's a Siffert?
I think a minimum standard to be called a "siffert" is an autavia model with a white face and blue hands. Personally I believe only a cal11 or cal12 autavia with a white face/blue hands is a "siffert". But it's a matter of faith rather than facts really. If you are a true pedant the only siffert is the one worn by siffert...

5) The price guide says they are worth $2500-3000, would
you agree?
I haven't seen many people on the forum disagreeing. The price of "steve mcqueen" monacos has exploded in the past months, you never know, sifferts could be next...

6) If there's anything you want to know about vintage Alfa Romeos, maybe I can help.
I think the guys in the forum would get a kick out of seeing some nice old alfa romeo photos...I dont have any vintage cars, but if I was to buy one in reality it would probably be an alfa (in dreamland an original muira, but I don't see it happening soon).

shaun