There are two water channels in the bottom on the Octagon 20 Eco - you generally fill one channel during the incubation period, and both channels for the last three days while they're hatching. You'll need to get a hygrometer and monitor the humidity level.
If you don't get the auto-turner, you just turn the whole incubator by hand 3-7 times a day. It's super-quick and easy; however, if you're not home to rotate the 'bator, the auto-turning cradle is a life-saver - and it will probably increase your hatch rates. In my opinion, it's worth the extra money - especially if you're incubating valuable eggs.
I have two Octagon 20 Eco's and one auto-turning cradle. I'm planning on using one as an incubator and one as a hatcher this year - but right now, I have chicken eggs being auto-turned in one and duck eggs that I'm hand-turning in the other!
You might also consider looking at the
Brinsea Mini Advance incubator - it only holds around 7 eggs, but people have wonderful things to say about it. Personally, I think it's a bit dinky - it would be great for my 7 and 10 year-old sisters, but I'm sticking with the Octagon 20's. With the auto-turning cradle, you set your eggs and forget them, there's no worrying. The 'bator holds the temperature perfectly, and you just have to check the humidity and add water to the channels every once in awhile. I definately recommend the
Brinsea Octagon 20, but ANY
Brinsea incubator's a good choice (but, maybe stay away from the older Octagon 10's - I've heard some not-so-nice things about them).