Dvdo mini - if you are not considering calibration which you should, mini can do almost everything more expensive processors can do
Dvdo duo - little more control over deinterlacing and allow you calibration on 2d video
Lumagen mini - everything duo does and much advance calibration which works on 3d as well. Dvdo units have severe ringing(halos around sharp transitions), dvdo mini has ringing filter, lumagen units are natively immune
Calibration gives you the most improvement. Incorrect colors and bad contrast is more annoying then a few deinterlacing and scaling artifact(if you mainly watch blurays you won't have these issues)
If your budget is limited, you might want to invest the extra cash on display. Better displays have multi point calibration options available although not as good as the one lumagen units. If the display has inherit motion issues, none defeatable motion processing, these issues may not be remedied with external processing.
Since they stopped making plasmas, I don't even read about tvs anymore. So I couldn't help you on tv selection but the good news is there is way more to choose from when it comes to tvs. Video processor options are limited to few especially on budget.