Calcultions to choose between a transistor (switch)

You are asking what transistor is good for a low side switch that must be able to handle 1A and 12V. This needs to be driven from a CMOS digital output. You say you have no idea what the current output capability of the digital chip is (4013), but that's silly. Surely this is specified in the datasheet. For sake of example only (I didn't look it up, that's you're job especially since you didn't supply a easy link to the datasheet), let's say we don't want to draw more than 5mA from the digital output when it is high. I just noticed that these chips are powered from 12V, so let's say "high" means at least 10V. Again, this is something you need to look up.

A NPN transistor makes a simple and cheap low side switch. It has to be able to handle 1A. That is getting into the low end of the "power" transistor category, so let's say you can find one with a gain of 20 at 1A. That means it will need 1A/20 = 50mA base current minimum. Since that's more than the digital output can source, you need another transistor in there to provide more gain. This first transistor (since the signal goes thru it before the power transistor) only needs to handle a bit over 50mA and 12V. That's "small signal" territory where much higher gains can be had. There will be many cheap transistors that can be counted on to have a gain of 50 in this case, so the digital output now only needs to source 1mA, which is within spec. Here is one way to arrange all this:

One problem with this is that since you are using such a high voltage for the digital chip, the base current for Q2 has to drop accross a higher voltage and therefore dissipate much more power than if the digital output were a more normal 5V or 3.3V when high. There are ways to deal with that, like a divider right after the digital output, but I'm showing the conceptually simple case. For the purpose of computing the dissipation in R1, you have to assume the digital output goes all the way to it's supply, which is 12V. That 12V minus the two B-E drops will be about 10.6V. (10.6V)^2 / 160Ω = 700mW. That's quite a lot and means you have to use at least a 1W resistor.

As I said, there are ways to mitigate the wasted power in driving a NPN low side switch, but this application is really crying out for a FET. Now the 12V digital signal is actually a advantage. There are many many N channel MOSFETs that can handle 12V and turn on to a small fraction of a Ohm with 12V gate drive. This simplifies the drive circuitry, and is what I recommend:

Let's do a quick sanity check on the power dissipation of Q1. Let's say the FET goes down to 100mΩ. (1A)^2 * 100mΩ = 100mW. That's something even a SOT-23 package can handle, and 100mΩ is quite high for a FET of this voltage and current rating.

As for reversing direction, that's just how the coils are sequenced. If you produced the coil signals with a microcontroller as we've been telling you to do all along, this would be a trivial thing to do in firmware.

When choosing the right transistor for this job, first I'll eliminate the PNP transistors. They're a bit more complicated to use in your case. As you said, for a PNP transistor, active high becomes active low, meaning the transistor will switch on when you apply 0V from your Arduino, but it won't switch off when you apply 5V from the Arduino. You'll need to apply 12V to the base of the PNP transistor to switch off (VEB = 0).

Leaving PNP's behind, looking at the NPN's that you have availabe, only the BC547B (Ic = 100mA) couldn't handle the 480mA current that your siren needs. From the remaining 3 transistors, I'd choose the one that can handle the most current, just to be on the safe side. That would be the BC517 darlington, which can handle a maximum of 1.2A, more than enough for your siren.

Only now you'll have to worry about the gain of the BC517. But, because BC517 is a darlington transistor, it has a huge gain (hFE = 30,000), so you can easily switch on the transistor with a very small base current. If you chose to drive the base of the transistor with a 1KOhm resistor, you'll have a 3.6mA base current, which is sufficient for your purposes.

So the winner would be the BC517.