Which guidescope to buy?

I have a 12" LX200 standard and want to buy a guidescope that is compatible with the LX200 drive.

As I'm new to astrophotgraphy, the question is what is a reliable guidescope that will track well enough for 10-20 min. exposures?

You can use just about any scope for a guide scope. The key is focal length. Meade recommends that your guide scope focal length is at least half the focal length of the scope your imageing thruogh. Although, as Michel pointed out, you can guide through your finder scope wich the focal length is less than 1/10 of the OTA. SBIG makes a FR237 focal reducer that comes with a 4" long tube that makes a 100mm scope that they say can be used for guiding once attached to a ST5 or simular camera. Bottom line is I don't think you can go wrong with any choice you make.