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it does not matter which version of SL you buy. U just do an update after the install and it will bring u up to the latest., so 10.6 or 10.6.3 would be fine.

I would do a clean install. That always seems best.

You want the retail disk. You do not want the disks that came with the mac when it was purchased because those are specific to the model that was purchased.
 
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