Depending on where you are staying it may be a good idea to purchase a T-casual card at one of the Metro stops--we purchased a 10 trip card and used it on the metro, bus, and even the Rodales R2 train from the Sants rail station to the Prat airport!
We looked at the Barcelona card and it really didn't offer much for the price. as far as getting tickets for the various sites like Sagrada Familia, Park Guell, etc. it's best to go to the actual genuine real venue sites and get tickets directly from them. They probably won't be selling tickets for October yet, some sites start opening up their dates 2-3 months in advance. Not sure how busy things will be in October, but for May and June the tickets seemed to sell out a week or two in advance.