A/V Master 57 has provided you with good and detailed guidelines, I shall supplement with your specifics.
1. Modern plasmas, and par your intended Panasonic model, handle burn-in issues sensibly.
2. 120Hz is not of a big deal for your situation. If you think you like to wait for the price to fall, it is not so much waiting for 120Hz, but for the better models SONY XBR4 and Samsung 71, 81 series. However, there is always something newer with some newer technical features. There is already the XBR5. One point or another you hop in.
3. You won't be happy with the 61, when besides HD you want to play your regular DVDs (although upscaling helps) and watch SD channels.
4. If burn-in is your most concerned agenda, have you considered the Samsung equivalent 4264/4254 plasma? Samsung has this burn-in wipe-out feature. Samsung plasmas have own drawbacks - this is from Master 57.