I'd go for a simple retrim kit for the original seats. Over time the foam degrades and falls apart resulting in lumpy, unsupportive seat cushions. The outer covers on mine were in a good state so I just replaced the base foams, but replacement covers are available as well. If you're looking to switch to leather covers this could be a good option. Once refurbished, the original seats are actually pretty good. Also, access to the battery isn't impeded and it keeps everything standard.
The forward-facing rear seats from a 2007-16 model won't fit properly as the wheel arch boxes are a different shape. This allows the rear seats to fit far enough apart to get to them. If you fit forward-facing seats in an older model, they usually end up too close together. This means you have to fold one up to get in and out. Seeing as they are usually fitted with young people in mind, I fail to see how this is acceptable from a safety perspective.