Also, 50k Ohms as impedance is very high for output impedance. You want maybe 1k as output impedance, or better (50 Ohm is something to shoot for.)
Ideally, you build an active notch filter for separating 50 kHz signals, using an opamp or two. After that, you can use a low-pass and a Schmitt trigger (perhaps also a comparator to boost the signal) to clean it up.
I would not use a transistor amplifier at all; rather I'd use an opamp with say 100x gain (so 5 MHz gain*bandwidth product) and bias it down to filter out ambient. An automatic gain circuit could make this more robust to changes in ambient lighting, but perhaps you're not wanting to go quite that far :-)
Active bandpass (peak) filter design: http://www.electronics-tutorials.ws/filter/filter_7.html
Of course, a different option is to use a fast comparator, and a small MCU, and do all the signal detection/processing in the digital domain. But perhaps that's cheating :-)