Yes, all Barcelona city and a bit more of the metropolitan area, included the airport, is at Zone 1. You must check the zones for inter-city journeys, but for inner city ones you don't care.
Thanks. Can the T1 be used for the trip to/from the airport? Can you purchase T1 tickets at the airport?
Yes and yes.
In T1, from the tobacconist (Tabac) - cash only, I think. In T2 at the railway station.
There will be 5 of us with luggage, would you take the metro to Carrer Marti or a taxi?
About 6 Euros each in a taxi. I'd take a taxi.
1. Airport metro is not recommended as it does not reach the center and has pretty lousy changes. Plus, the only thing you cannot use the T-10 for is the airport metro stations.
2. Any other integrated public transport is cheap, as with the T-10 it's 1.02€ per person, but it will require changes (the exact rout depends on the exact location in Carrer de Martí).
3. A taxi is the most comfortable way and with 5 people it won't be expensive, as pandachris2 said. Just ask for a big taxi from the taxi marshal at the airport and be aware that it will probably be a bit more than 6€ each as you will, probably, pay for luggage (1€ for each suitcase larger than 55 x 40 x 20 cm) and a 3.10€ supplement for a large taxi with 5-6 passengers (this supplement is per ride, not per passenger). In addition there is a 3.10€ airport fee, which is included in the 30€ estimate but, as all supplements, will be added at the end of the ride so if the meter shows 25€ remember that the driver will manually add the supplements, so he´s not trying to con you (it's 3.10€ airport fee + 3.10€ large taxi = 6.20€ + 1€ for each large suitcase.
Great information, thank you so much.