How much should I pay for a 2001 Thiel CS 3.6?

Have you considered something like the Vandersteen 2's?

Agree with Philojet, unless you intend on blasting the Thiels in a large room. I have owned a couple of Thiel speakers and they do have tipped up highs. Have also owned the NAD C352 and did feel it had a full-warm sound which may tame the Thiels.
If you like the Thiel sound and the price, I say go for them.
Definitely would not worry about their age.

Buy them, they are classics, I had those in my system for 10+ years. Be patient, upgrade upstream slowly and they will give you years of enjoyment.

http://www.stereophile.com/content/thiel-cs36-loudspeaker-measurements

I used the 3.6 for about 8 years, and it is a very good speaker in many ways. It is quite demanding in amplifiers though. If you go with a solid-state amp, you will find that you need lots of power. I started with a Classe DR-10 with about 200 watts at 4 ohms. It tended to sound too restrained, like it was holding back, so I traded up to a Classe CA-300 with 600 watts at 4 ohms. It sounded much more powerful but even with those reserves the 3.6 still sounded like it was holding back dynamically.

I ended up trying a number of tube amps ranging from Cary CAD-100 monos to push-pull 300B monos, but in many ways the best sound I ever got out of the Thiels was with a bone-stock Dyna ST-70. Sure it wouldn't play as loud as the big Classe but the overall tonality and dynamics within its power range was superior.

So the Thiels are very good but don't be surprised if you start getting the urge to change other equipment to make them sound fuller and more dynamic.