Congratulations and welcome to the "Beauty On Water"!
I'll try to answer your questions!
1) Blocket is definitely the place to look. I would say it has more or less 100% of the ads for used cars. If you are looking for an Alfa Romeo it can aslo be a good idea to look at the ads on the two Swedish Alfa forums:
(The Swedish Alfa Romeo Club)
(Independent Alfa forum)
2) It's good manners not to argue about the price until you have seen the car. If you but in an ad trying to sell a used car for say up to 30 000kr you get tons of calls and emails from people offering crap money or asking for "your last price". Be polite, interested, ask relevant questions and keep the arguing until your face to face with the seller.
3) Insurances is a djungel. It's impossible to say which company is best for you. It depends on how old you are, where and how you live, how long you have had your licens etc. You need to contact them and get the prices for the car yo are interested in.
There are some websites that can help you compare companies:
Also you need to think about how much you r insurance will cover. Liability insurance (trafikförsäkring) is mandatory. On top of that you can also add theft, fire and glass (Delkasko or halvförsäkring). And as an additional step: crash insurance that covers your car in case you are causing the accident (helförsäkring).
4) I have a GTV TS myself and the road tax is 2199 SEK.
5) During summer there's usually a weekly meeting in the Stockholm area with the Swedish Alfa Romeo Club. More info on the website www.alfaromeo.org
You can become a member on the website without beeing a paying member of the club. Just register on the site.
One thing to notice is that winter tyres are mandatory in Sweden from 1st December to 31st March. If you buy a car and don't get any winter tyres with it, you must get them somewhere else. Blocket is the place if you are looking for used winter tyres.
Just remember that the traffic situation in Stockholm isn't the best. It's also very sensitive to accidents since there really aren't much alternatives if one of the bigger roads is blocked. Depending on where you live and where you work, the underground, local train or bus might very well be the fastest way of commuting.
Hope that was of some help. See you on the roads of Stockhollm! :-)
Oh, almost forgot: Sweden VS England