Sorry, I didn't see this earlier.
A quick check of the datasheet shows the 2N7000 good for 60 volts, so it should be fine.
The npn only ever sees the gate turn-on voltage of the fet, so really, it's voltage rating is not related to the supply voltage.
Any common NPN transistor will work fine.
The only rating you may need to watch is the dissipation of the 2N7000, which is 400mW. Assuming your 48 volts is a battery, the peak voltage at equalisation is going to be approaching 60 volts. To work the numbers, we'll use worst case and say it is 60 volts.
12 leds in series at Vf = 3.5 volts gives a total of 42 volts. Take this away from 60 volts, gives us 18 volts across the fet. If you ran the leds at the full spec sheet current of 25mA, dissipation in the fet would be 18 * 25 = 450mW, exceeding the devices rating.
If you take the common-sense route, and run the leds at slightly less than max current, say 20mA, now your dissipation comes down to 360mW.
Considering this is absolute worst case, I'd say it was doable. Just be aware when you build it, that the fet should have free flow of air around it for the worst case situation. Even consider a small heatsink. A square inch or so of sheet aluminium.
Hope this helps.
Amanda