Man 20ft is far and your gonna lose some image pop, if its on the ceiling why cant you mount it closer? I would mount (from my 4 projectors of my own and many others done for friends) around 12-15 ft from screen but thats me.
If you are going to get a roll up screen do make sure it is a "Tab Tensioned" model or in short time your likely to get a wave in your screen............not pleasing to the eyes.
Go to Projectorcentral.com or google that along with "throw calculator" and examine those specs for any model you may want to buy.
You can get a Panasonic 4000 (great buget unit for around $2000) but if you just want to enjoy occasional music and such even a used 720p model will work perfectly and can be had for way under $1000. Dont let anyone convince you 1080 is this vast jump over 720p, its just not true. And this is from a guy who owns a $10,000 1080p LcOS unit. 720 will do wonders for you and you got tons of cash for concert videos or music.
I am glad to help if you want to shoot me a mail. Cheers