I haven't gotten around to it yet, but I have picked up quite a few for repaints. I can tell you from experience making custom 3.75" astromechs that it's not easy. You'll need to practice quite a bit before you get something that's on par with Hasbro's quality.
As for paints, I always used the cheapo acrylic (DO NOT USE ENAMEL) paints from the craft section at Walmart. I don't think they carry the same brand anymore, but it used to be Folk Art at about $1 a bottle. I'd get the cheap brushes as well, from the same aisle. You'd definitely want to seal it afterwards. I used Testors dullcote that I had to order from their website. I never had a problem with chipping, except for paint rub from the ball joints, which isn't going to be a problem here.
No need to take them apart IMO, but if you feel the need, you can use the same technique that I used to use on the smaller guys. Go to the bathroom or some other smaller room. Take the figure and repeatedly throw it on the ground until the dome opens up. You'll want to be in a small room to make finding the small parts inside a lot easier as they tend to fly all over the place. Maybe not the best method, but it did work for me and never damaged the figure in the process. I don't know how that would work with these, so try at your own risk. I suppose you could try using an xacto knife to cut around the seam, but you'll probably end up either cutting your finger or making a new seam.
This made me laugh. I picture you in a racquet ball court smacking R2 around with a ping pong paddle.