My comment on males and females - entirely up to each individual dog and breed. I have had some incredibly high drive females - my best agility dog by far was a female, that dog was up for any kind of action and still is at age 11. I currently have a lower key laid back male who doesnt do playing, and a very high drive playful male. Same with the girls I currently own. My preference is for females, I find they are often more focussed younger for what I do. But the boys are good too.
As to breed and length of time leaving them, I think that is up to your management. I have had cattle dogs all my life and have also worked long hours having to leave dogs on their on. The rules for this is to exercise them for a good 45 -60 minutes before leaving in the morning. Even if you have to get up really early and regardless of what the weather is. Good running and playing exercise too, not just walking round the block.
The same applies when you get back from work. I set aside an hour and half or more per day. On the weekends I did much more and my dog went everywhere with me.
So just be aware of this with high energy breeds that you are planning to leave in your yard for long periods of time. I found marrow bones to be helpful too. Some young staffies I know are pretty full on but I also know some that if exercised and trained are fine with being left all day. I do know a staffy lab cross and he is very very full on and a staffy GSD X that is more laid back. Again depends on the dog.
I think rescue is a good place to start. Just make sure you know if there are any issues. I have 2 from rescue and one in particular is timid but he has blossomed into a very nice dog. Sometimes high energy rescue dogs like one of mine are dumped because of this but in the right setting these problems dissapear, like with my kelpie when she came to me.
Good luck and take your time.