Cervelo P2-SL value

It should still be 10 speed Ultregra, make sure it's not 9 speed (for some reason, 9 speed Ultegra and Dura-Ace pretty much sucked…).

If it's really well maintained with newer chain/cassette/chain rings/brake pads/cables/housing, then $700 is ok (not great, but ok). The P2-SL is a very fast bike, even by todays standard. It's not overly forgiving though, so if you race on really rough roads, that may be an issue (although that can somewhat be negated by using 700x25's instead of the more common 23's).

Ya its the 10 speed, it has the 650 tires on it.

Ok, so 10 speed is a plus and makes the asking price within reason. 650's are great, especially for smaller frames, but they are less common, so finding a tube in a pinch may be a bit of a challenge (although you can always order an extra tube just to have it around…) and you will have less choice of tires. Wheels are kind of interesting. Since they are less common, you will have less choices (although all the big brands do make 650 wheels, so not much of an issue), and you will have fewer used wheels for sale. The good news is that since they are fewer buyers, then tend to be less expensive when you buy then used.

I've only used 8 speed, 9 speed and 11 speed myself (somehow I missed the 7800/6600/5600 series), but my research indicated quite the opposite. All groups have their strengths and weaknesses, but a few reviews online say the 10 speed group was, in some ways, a step in the wrong direction, that was "fixed" with the 11 speed group.

Everyone has an opinion, and lots of online reviews are biased, so I take them with a grain of salt, but I like the 9 speed group so much that I've pretty much standardized it across all my "pavement" bikes; 2 tri bikes and 2 road bikes, with almost enough parts to "upgrade" a third, my older QR Kilo, from 8 speed to 9 speed.

My limited experience: While the D/A 7700 Bottom Bracket is a little sensitive and harder to set up, the Ultegra or 105 BB is a nearly bulletproof replacement. Some replacement parts are expensive, but things like cassettes can be found dirt cheap on Ebay. Shifting is smooth as butter with both DA and Ultegra, and everything has been very reliable for me. I have nothing against the newer (or older) groups, just wanted to speak in defense of the 9 speed groups, as they've been nothing but good to me.