Welcome lexm.
I have the very bike you speak of. A complete default s/s Brompton,not one extra bit ordered with it. Naked as a j bird. Love it. Its officially a S1E,only one in Canada Brompton tells me,most order with at least 2spd. I did swap out the rear cog with a 13t Surly cog which fits right on. That gives me about 66 gear inches. Originally it was a 12t cog and 54 chainring. Id suggest keeping it in the low to mid sixtys for gear inches and knees should be fine. Im 64 and knees are still good. If i have to hammer too hard because of hills or what have you i just get off and walk,I call it "cross training". Thats my story and sticking to it.