OK, here are some basics then:
Specs:
If the datasheet says voltage between base and emitter is 6 volts, then it must be speaking about absolute maximum ratings. These ratings are the no-not-exceed numbers or else. During normal operation transistor Vbe is very close to a diode forward bias voltage of ~0.6 volts maybe a bit more. This voltage is our target here as well, you can forget the breakdown limit of 6 V. The other significant max rating is the VCEmax that must not be approached too closely. In this kind of low voltage circuit where the decision is not costly, i would leave a considerable margin and select something tolerating about double the needed voltage. That gives plenty of headroom for sporadic small spikes or similar disturbances. So, pick one with VCEmax about 60 volts or so.
The next important spec is max continuous collector current ICmax. Again, this datasheet figure should be comfortably larger that your expected maximum current. Most power devices will tolerate short surges well above the continuous rating, but you as the designer need to be aware if such is the case and design accordingly. Here the temporary max current is 2.4 amps. The continuous current will be lower, but as you don't know exactly how long you need to operate on the limit, best specify for continuous 2.4 amp current. Again, not a costly decision for this particular device.
The next item to check is what is called the safe operating area. This diagram outlines the borders for Vce vs Ic, inside of which the transistor operating point must stay in order to avoid the destructive phenomenon of second breakdown. For this kind of application you will want to use the DC curve, i.e. continuous state.
The final item for a quick selection is to check the large signal current gain of the potential device. This is noted in the datasheet as the hFE. for small amp transistors it can run into hundreds but for a large power device it could be as low as 10. This is the ratio between base current and corresponding collector current. Usually hFE decreases with increasing Ic so you should check the datasheet current for the given hFE.
On to the current limiter:
This is an archetypical linear regulator that is reliable and performs OK for you if it is constructed correctly. The main drawback of this kind of circuit is that the regulation takes place by dissipating all of the excess power as heat. You can use the emitter circuit of either of the linked circuits, calculations are the same for both. The 2 transistor circuit will give a more precise limit, otherwise they are more or less equal.
Basic equations:
Vin Supply DC voltage into the limiter circuit (RMS value if there is significant ripple)
Vout Output voltage exiting the limiter circuit. If you use average values, you get average dissipation. Instantaneous values give respective instant values of course
Iout Output current exiting the limiter. Logic same as above
Re Resistance of the pass transistor emitter resistor
Vre Voltage across the emitter resistor. We want 0.6 - 0.7 volts at the current limit to match the transistor Vbe
Vre = Re * Iout ; Vre --> 0.7V @ 2.4 A ; Re = 0.7 / 2.4 = 0.291 ohms
An important design item to verify is the power loss in the pass transistor. The formula for that is:
Pd = (Vin - Vout) * Iout - Re * Iout^2
You don't say what the supply voltage to the limiter ´circuit is, but assuming say 40 V, the numbers become:
Pd = (40 - 30) * 2.4 - 0.291 * 2.4 * 2.4 = 22,3 W
This is a considerable power loss and the pass transistor cannot survive without proper heat sink. Now this loss is not continuous and you can re-calculate for the continuous current, and you will find out that the power loss with 1 A current is just under 10 W. Assuming your heat sink has good thermal capacity, you can dimension it using the latter figure, provided you don't do repetitive starts too much.
The power transistor can be a jellybean TO-220 packaged type that is easy to heatsink. Subject to the hFE of the selected component, the auxiliary transistor can be a 2N2222 or similar.
Power dissipation in the emitter resistor is
Pdre = Re * Iout^2 = 0.291 * 2.4 * 2.4 = 1,67 W, so select a 3W or 5W resistor and it will not be burning hot.
Looking at Digikey component selector, we get several candidates for the pass transistor. in the 60-100V range there are the trusty old TIPxx devices, many of which will do what you need but they have a rather low hFE of 10-15 at higher currents. They can be used in the 2 transistor circuit though, since the aux transistor will mitigate the lack of hFE by its own amplification. The TIP41 has a SOA (safe operating area) DC value of 6 amps @ Vce=10V. So it will not suffer second breakdown at these power levels.
Another alternative would be the TO3 packaged BUX10 power device targeted for industrial and military applications. It will dissipate max 150 watts which is of course gross overkill, but the package favors more efficient cooling. It will also maintain a hFE of min 20 at 10 amps. But there are others and based on the above you should be able to make your choice.