For the money, and best performance out of both for the money, I suggest the traditional audiophile approach. This is basically keeping a 2 channel setup/rig, while looping a good 5.1 processor into an auxilary or "pass through" input on your 2 channel preamp, as another "source input"!
What this will do is to allow you to have the purity of your 2 channel system intact, as usual. Then, you can simply switch to your input that has the 5.1 processor attached (where all your other channels/speakers are attached to your AV pre/pro), playing the left and right speakers out through your 2 channel setup at unitygain.
Again, you can have your 5.1 pre "off" while doing 2 channel sources, tuner, cd player (assuming quality CD source component,dac's, etc), turntable, etc). This is the best compromise for the money. You can get a superb sounding higher end 2 channel pre for $500-700 range used, often less, and a good Acurus Act 3 (superb for 5.1 movies,dd/DTS,etc) for around $400-500 range used. This is how I'd do a system for a budget, where 90% of your needs are 2 channel, yes!
Otherwise, you're looking at somewhat of a compromise most often in the 2 channel department, greatly leaning on the quality of the preamp section and DAC's in your AV processor! Often, you need to spend lots to try to get high fidelity 2 channel PCM from a pre/pro, as compared to a good 2 channel analog setup.
If you don't want to do this, you might look into used Anthem AVM20. Still, I bet you won't get the same 2 channel sound as what I recommend, with a good CD player/source.
Things are always a compromise one way or another...convenience vs perfomance
