After some consideration I am now thinking that a simple "crowd funding" AT could be a better 3rd use case (I was considering an "auction" but it seems to me the VS could probably handle that with some very minor modifications).
The idea would be that this AT would check its balance after n blocks (so it would be a good candidate to use the "sleep until block X") and if it is >= X then it would send all its funds to account Y (being the "project's account") otherwise it would iterate through all the txs it had received and refund the amount back to the sender (minus a small amount to run itself).
What do you guys think?
Do i get this right, everytime a new transaction comes in, the AT is beeing executed from the beginning, but the memory is at the state where it ended last time?
Pretty much although it actually depends how it had halted - if it had finished running (or reached a "finish" op) then yes it does that but if it hit a "stop" op (or it was halted due to lack of funds) then it continues from that point rather than from the start.
From where is execution time purchased from the ATs XNTs or from a fee in the incoming transaction?
The execution time is in accordance with the AT's balance although a fee (possible mandatory) will cover a small amount of execution (so an AM would actually trigger execution).