Greetings Mario,
There are many people much more qualified to answer your question here, but I'll give it a shot anyway since my tastes and expiences somewhat echo your own.
I currently own SR-007Mk2's and have have owned 303's in the past. I have heard a lot of 007Mk 1's. The Mk 1 has a subjectively more even and neutral overall presentation. The Mk 1 responds best with solid-state or very neutral tube amplification. The Mk 2 has a bit of a mid bass bump and a slightly more forward presentation. The Mk 1 is preferred by most here, but the Mk 2 is a very fine headphone in it's own right. With some amplification (primarily tube) the Mk 1 can sound a little recessed and dark. I build tube amplification and listen to rock music. In my system I actually prefer the Mk 2. I also prefer the Mk 2 to the LCD-3 ( also a very fine headphone), which I also own.
I was very fortunate to get a one time shot at a brand new U.S. Mk 2 with warranty at essentally a used price. Like you, I was hesitant to purchase a 10 year old headphone in unknown condition for the kind of money we're talking here. Many of the Mk 1's I've seen at meets are starting to show their age. If you ever need new drivers from Stax, they will be M 2's. If I had to pay full list priceI for the Mk 2's i might have made a different decision, but I have no regrets and greatly enjoy my Mk 2's.