I think it's a week to activate. I also think that if you buy an Oyster online with extra $TOPUP, it gets posted to you with the topup already activated.
I don't know if you can do things like add the railcard discount or setting up auto-topup (which given your comments about banking security I guess you wouldn't want) before you've physically got the card though. (I recently had to get a replacement Oyster - I got them to activate the auto-topup before sending it out, but suspect this may only be possible on a replacement card not on a first issue. I also did it through the callcentre rather than online.)
TBH, if you're going to be starting at an Underground station with open ticket office anyway, I'd be inclined to just get the Oyster card from the ticket window anyway: that way you can put exactly as much money as you want on it, and get them to do the railcard discount too.