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Doesn't matter if your Newtonian is f/5 or f/10, using appropriate eyepieces and/or barlows will give as good views of the planets as an f/10 SCT. My 8" f/4 newt gives me as good views as my C8 at the same magnification.
Slow newtonians are not really made anymore. You really don't want to carry around a 12" f/10 newtonian with a 3m long tube to accomodate the focal length. The mount would need to be overengineered too.
You can make your own mirror. Not overly difficult, and there is a thread on this in the DIY forum. Have a look. A slow mirror is much easier to make than a fast one too.
Mental.
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