Kai I agree on the added isolation, but I think you will not get the collector of the bc550B below about .9 volt

Hallo Terry,

my databook says: Uce = 0.2V for Ic = 100mA, Uce = 0.08V for Ic = 10mA, when Ic / Ib = 20. So, 0.2V plus 0.47V of charging via 1M resistor gives 0.67V, which is still below 0.8V.

But I think you are totally right, when telling that BC550B is not the champ for providing low Uce voltages! Well, in that case, BC337-25 may be used, which gives much lower Uce.

By the way, typical gate threshold voltage for turning-on of BS170 to Id = 1mA is 2.0V. The guaranteed range is 0.8V...3V. So, may be that some selection of BS170 might be appropriate?
Instead of BS170, also a darlington combination of two BC550B can be used, Then, 'threshold voltage' is bout 1.4V, while also needing very small base current.

Again, you are right, Uce has to be kept in mind! And, there's always needed some testing. Think only about possible different specifications of transformer...

Regards,
Kai