Most torrent software will buffer files to local storage so you might not get it off the SSD even if you specify writing to an external space. Adding lots of RAM will allow files to stay in RAM while mapped in virtual memory and reduce the IO to the SSD on a theoretical level.
In pracice, I'd say get the SSD you like - it should have a 3-4 year OEM warranty or you can buy up AppleCare if you feel you have any chance of wearing out the drive. You'll have maybe spend a few $$ in exchange for driving your hot new mac. If you feel it will wear out soon, get a service plan that you like. It would be like not buying a new hot rod since you might need a new clutch if you drive it hard. You might, you might not, there's only one way to find out.