Which type of Hong Kong Octopus Card do i buy?

hi,my friend and i will be travelling to HK but we only have 1 day to walk around.
i had actually planned to take the MTR from airport to Wan Chai Station,then to Tsim Sha Tsui and then switch to take the normal star ferry to Central before taking the MTR again back to the airport.

as such,i would like to ask:

1) which type of octopus card do i buy? if i get the airport express card,do i opt for single ($250) or double
journey($350)? does the single trip include back and forth journey from the airport?

2) do i need to top up extra value into the card since i am taking the ferry?
if so,do i top up at the airport or at the ferry terminal? can the ferry tix be purchase by cash too?

thank you in advance :)