With all plasma technology implemented on HDTVs, a very, very small minority are able to notice the flicker. Luckily, I am not one of those persons.
Count on the S60 to have a significantly poorer PQ than what you have on your ST60. Like I said, it's the best one on the market since the Pioneer Kuro craze (use google for more info).
See my previous post for referrals to one of the other (non-Panasonic) plasmas that might work for you in a dark space. I completely understand about what you are after to compliment your viewing needs, especially non-daytime 
If you have the $$, you could hedge on the future value of the ST60, since it is no longer produced and is super high quality for videophiles. IOW, buy either the F5300 or 8500 for less and wait a year to put your ST60 up for sale. It will likely sell for more than you paid, or close to that number. YMMV.