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Something like that will be a huge amount of work.... You'll need a completely new window winder setup, as the existing one is very reliant on the full height frame to support the glass. I'd recommend tying to find some frameless doors in a scrapyard (perhaps from a Subaru?), so that you can see how they're done, and whether or not those parts can be used in the Volvo doors - if they can, this will make life a lot easier! However you'll also have the problem of getting the rear windows to wind down far, due to the wheelarch.

If you were talking about a 2 door Amazon (and wanting to remove the centre pillar too), I'd say it could be a great idea, but for the estate, I'm not convinced that the work required will really produce a look that is worth the effort. However if you think it can, and you have the ability to do such a conversion, then go for it!