it is a very good idea to have an extra siren, inside the house, for example fixed to the landing ceiling where an intruder can't get at it without a stepladder, and the noise will make him anxious and hopefully run off
There is no limit to the number of sirens you can use with the HSA6400 panel, because it just transmits one siren signal, and does not know, or care, how many sirens have been learned in to receive it. Or in a large house you can fix one to the back wall, near your bedroom window (you may then be able to lean out of the window to get it when you change the batteries).
The control panel can be kept upstairs, near or in your bedroom, if you have an extra keypad somewhere near the front door. Otherwise you might not wake up during a break-in. This also means it is out of sight and out of reach of an intruder who might pull out the phone line.
Unless you have quite a small house, the cheapest way to get accessories is to buy the Standard kit, which has a siren, a keypad, two PIRs and two door sensors (but no control panel) for about £99. The accessories are available individually if you prefer.