Yep ross cobbs are good and also cornish crosses if you can get hold of some of these it would be worth it
If you dont want too much fat, give them a natural life with lots of free ranging and minimal treats.
Commercial birds are slaughtered at between 4 and 7 weeks but it depends how tender and tasty you want the meat. A good time to do cornish crosses is roughly 8-9 weeks fed on a finisher diet for 2 weeks before slaughter.
Also a tip from negative experience, make sure the crop is empty before you process them, i.e dont feed them for 24 hrs before slaughter, otherwise if you rupture the crop during processing you will be sprayed with a very smelly substance, and trust me, the smell hangs around for a long time.

Different slaughter times and when the meat is ' ready' depends on the breed, how fast or slow the birds grow, and their diet.
Best of luck, and excuse the long message

abc123