Hi!
I may say that I am in the same boat (my wife also has knee problem, and we also decided to use Dahon Briza + TSDZ2 350W motor). Now about the motor - Last year I put 750W TSDZ2 on my city bike and I don't feel it is anemic. With 8-speed casette I can climb the steepest hils with ease, and I am not exactly lightweight myself (about 105 kg). On flat terrain, when derestricted, it will go a bit over 40 km/h, but it will require some effort on the rider as well. Optimal cruise speed is about 30-35 km/h, with minimal effort from the rider.
People often feel it anemic (compared with BBS) because it is torque sensing, and it always requires some pedal pressure to make it work. On the plus side, the battery range is better, for the same reason.
So I don't think that 350W motor will be too weak either.
For the Dahon Briza - I have differerent recomendation - just say "NO"

(at least when TSDZ2 is concerned).
Dahon itself is nice, lightweight, it rides better than 20" folders, but the bottom bracket design is really not suitable for this conversion. It is still the same standard 68 mm bottom bracket, but to fit the motor, I had to cut the bottom tube, remove the kickstand, also cut and remove pieces from both sides of the plate where kickstand was attached. Ewen with all this, it is not possible to fit the motor in one piece (I had to remove the lid on the left side first - and throw away the warranty for the motor :-( ). So i lost both warranties - for the motor and for the bike. Also the chain line is problematic, due to awkward geometry of the rear fork, the first gear cannot be used. And I had to design special torque arm, because standard arm does not fit.
And after all this trouble, my battery (ordered from Aliexpress) is stuck somewhere in the middle of the Corona crysis, so I was not able to test it yet :-(