We normally leave our British Toggenburg wethers until they are between 8 and 10 months old. The last lot killed out at 24.7, 24.2 and 22.7kgs respectively, they were 10 months old and just becoming a real nuisance....
Goats are probably a bit more expensive to rear (as they don't do outside grazing if it is wet) and will need concentrate to help finish them, but the meat is incredibly tender when cooked right. Ours were hang for 10 days.
Saanens may not get quite so big, but I would try it. You will need a shed or similar, and they will need hay when inside, I wouldn't worry too much about goat specific food and feed them on lamb/sheep mix, plus if you can collect lots of branches for them - hazel, willow, some ash they will love you even more.
I am surprised someone is selling them for 30 per head at 6 months - it cost them more to rear them for sure!