Re: The Cart before the Mule
I don't think you'll have any regrets with the a850. Make sure to check the firmware on it, as the update from last year did give it a nice minor boost in AF performance, especially in low light conditions with just about any lens. The update also makes the camera act a bit more sensible when using non-chipped lenses (think m42 adapter and lenses).
There will be some minor adjustments, some may be frustrating, jumping back and forth between the a77 and a850 when it comes to controls, but they are minor. I'm sure you'll like the quality of the raw images, as I do think they require less work than those of the a77 in most cases.
Regarding the lenses, which 50mm and 28-75mm are those? It's worth looking for the f1.4 rather than the common f1.7 50mm. Not that the f1.7 variant is bad, but it does require a bit more stopping down to get sharper images than the f1.4. I'm assuming the 28-75mm is the Minolta f2.8 D variant. Not a bad lens, but at least with my copy I needed to stop it down a bit to get consistently sharp results. I also had some erratic focusing issues with it, but again, i think it could have been my copy. I made the mistake of trying the 24-70mm Zeiss, and that ruined me. Between the SSM and sharpness near wide open, the 28-75mm didn't stand a chance. 3 times the price, but... Well, you know how it goes..
Anyway, enjoy the new kit, it's a nice one!
Re: The Cart before the Mule
The 50mm is 1.4 the 28-75 is the Sony SAL2875. I tried it on the a77. It's pretty good. Not slow but not fast either. It focuses real well with just indoor lighting. It seems pretty sharp. I am sure the CS blows it out of the waters. I have a CS 24mm 1.8 for the NEX and it's awesome, no doubt about it. I'll pick one up but that's down the road.