Power and cooling...
There are two important factors that will effectively limit your choice of processor: power and cooling. With a power budget of only 250 watts and a CPU cooler clearance of only 38mm, not all CPU's are supported.
If we subtract the 75 watts for the graphics card another 50 watts for the rest of the system, that leaves us with about 125 watts of power for the processor. While this is pretty generous for most processors, it rules out the highest end ones because they use more power. But with only the lowest profile CPU coolers supported, those would be too hard to cool anyway.