Welcome to the site. Its refreshing to see someone come here for environmental gains vs the usual cost issue. There are many things you can do to the Volvo to decrease environmental impact. I would suggest reading some of the stickied thread in the forums to get a better idea of what you'd like to do.
The manual transmission swap won't be an easy task, but shouldn't be incredibly hard. Price will depend on used parts avaliable in your area. I suggest using
Car-Part.com to find prices on transmissions and other misc parts you'll need. For reference, I have a DIY written on how to do a swap on a Toyota Starlet on my site
here. Its definitly not your vehicle, but it'll give you an idea of what you would need to do.