I wouldn't be so quick to judge the H5000. In my opinion, the GT50 and the H5000 is closer than you'd think. The Panny S60,U50,UT50,ST50,GT50 are in a similiar running field and the H5000 about equal to the S60, so contrast ratio wise, the H5000 is very competitive. (I've already had two people telling me they've sold their F8500 for the H5000 for superior black level) I'd hazard a guess the GT50 is still superior in calibration but the H5000 also has great calibration option for a budget plasma. It's very light on features compared to your GT50 though...no 3D, no SMART TV, no IR filter, etc. It's also using a pentile array that may not be ideal for you depending on your seating distance. One of the Panasonic S64 owner has gone for the H5000 and he feels it's actually an upgrade. Another guy who had the VT60 who went for it has said the downgrade is not really vast.
If you want to go used, definitely go with the ST60 as it has the best value.