You won't get a seat on the Overground at that time, but it won't be unbearably crowded either (plus the carriages are air conditioned and the journey to C Water relatively short).

You'll find the interchange with the Jubilee at C Water (otherwise a handsome, pleasant station) absurdly crowded given how close the Overg and Jubilee are at that point - the tip to go for the exit and then down to the Jubilee from the ticket hall is the lifesaver here.

Trains to London B aren't quite as frequent as the Overg but you do have a chance of a seat - they're usually much longer than the Overg ones. The interchange with the Tube at London B is long-winded, but better than it was. Bear in mind that LB is in the midst (and will be for the next half decade) of enormous works. But there's always the CW option.

I'd say it's a pretty good commute overall, especially given the rental/purchase prices.