I went from a TE610 to the TE250, but mine is a 2010 and is significantly different from the 2009 model (even lighter). As to the cost, it's worth what you're willing to spend
. Bikes are never a good investment, except perhaps emotionally.
Height-wise, the TE250s are all very tall bikes, just as tall as the 610. They are lighter though, and that always compensates somewhat.
As a dual-sport, be aware that they are high-strung race bikes, with a very close-ratio transmission, and not suitable for droning down the road like a WR250R. You can connect trails, but that is about it. If you plan on any highway miles, look elsewhere.
As to reliability, the 2009 models share the same bottom end as the bigger displacement models, have a decent oil capacity, and are therefore pretty much bulletproof. I would do oil changes every 1,000 miles or so if I was just doing trail riding and not flogging it too hard. Valves tend to settle in after 500 miles or so and not need too much adjustment after that. However like any small displacement thumpers, there will be a top-end rebuild in your future, so keep that in mind.