I'd go with 10 and 20 thousandths or what the British refer to as quarter and half choke. That's worked for them for over a century and with modern shotshells will work for you just fine. I prefer the red Pachmyar "Old English" pad for a hunting shotgun, but choose whatever works for you.
RST shotshells are top notch and run about $11.00 a box or $110.00 a flat plus delivery. If this is primarily a hunting gun, you'll find a flat lasts a long, long time. I've also used the Experiencia shotshells in several of my British sxs and like them a good deal. They can be ordered in loads as light as 3/4 ounce, although I usually buy the 7/8 ounce loads.
