At the risk of plugging my own Trance advert (again) here are few more thoughts on a good first full suss for "cross country and trail centres".
I think the ~120mm travel, ~69 degree head angle, 26" bike is a great place to start. It's manoeuvrable enough for XC and will easily cope with anything you'll find at a trail centre. If you find that you want more speed and can cope with any descent that you'd want to ride easily enough then you can go shorter, steeper and probably with a bigger wheel (100/70; Anthem, M109 etc). If you find yourself pulled more towards faster steeper and more technical descents and don't mind taking a bit longer on the climbs and sacrificing some low speed manoeuvrability then you can go a bit longer and slacker (140/67; Five, Heckler etc). But the 120/69 package is a good place to start.
That doesn't just mean the Trance of course as loads of bikes are built around that geometry e.g. older Trek Fuel and at the cheaper end things like the Boardman FS and some of the Rockrider bikes I think.
I think you probably do have to consider wheel size, if only because this is probably a bike that you'd sell on again in a few years as your riding becomes more specialized. We can rule out 650b as we are looking at second hand and I'd be a bit wary of 29ers. Not because there is anything wrong with them, but because they seem to have had their day in the sun and fashion is moving back to smaller wheels, which may make them harder to sell in a few years. It's all just guesswork though.