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Without a doubt you want the SBIG model with the built in off axis guider. I think its the STF (not sure of the model names).
If you don't have either self guiding, self guiding in the filter wheel or an off axis guider you are asking for trouble big time and will end up throwing the scope into the scrap heap it will be that annoying!
Guide scopes at that imaging focal length are going to be very tricky and an ED80 whilst a lovely APO scope has a weak, semi loose focuser which will not be up to the job.
I just installed an SBIG STi guider guide kit on my CDK17. I intend testing it first clear night to see if it will guide accurately at 3 metres focal length. it is very rigid even though its only a 100mm lens guide "scope".
I can let you know how it goes.
I have imaged semi successfully using a Vixen VMC 95 guide scope (over 1000mm focal length). I would sometimes get slightly eggy stars in some angles. With an offaxis guider I almost never do. And that was on a PME mount with PEC on.
Greg.
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