SRD7-Pro and Mk2 transformers are rare as balls. You basically have to get lucky, or watch the market like a hawk, or put up a WTB and take the grossly overpriced offers that you'll get... oh maybe once every 2 months or so. Most people that have them will not sell them for any reason whatsoever, even when they don't really use them.
Even if you do have one, the O2 needs a good speaker amp to shine. Remember, the SRD-7 doesn't have a voice of its own. All it will do is blur the details somewhat and loosen up the bass, since it doesn't have as much juice as it needs to in order to control it. But the rest is up to your amp. And since the O2 is so insanely revealing, then the amp has to be good. If it isn't, you'll know about it instantly.
Ditto for your source. The O2 needs a reference quality source. That isn't to say that you won't enjoy the O2 without one, but you'll hear all of your source's flaws instantly.
So, I don't really think the O2 is a very valid recommendation if you're not looking to sink a lot of money into your rig. When I got mine, I went through several amps and several expensive sources all in a few months just trying to get it to sound right. And, I'm not even sure if I'm done yet.
Can't help on the 202 vs 950 question since I don't really like the new Lambdas much, but also never heard the ESP950. I'd suggest auditioning stuff if you can.