Hi and welcome!
1)As I understand you're looking for horsfield.Horsfields do like digging so deeper substrate is the best.6-7 in should be enough in my opinion.
2)Sounds ok.If you have space then it could be even bigger.
3)Tortoises are solitary animals so they will be fine on their own.When babies then they can be kept more than one together but when grown ups then it could be a problem to keep them in peace.
4)Depends on breeder when babies are ready to go to new homes.Babies are pretty young.
5)Underbed storage box is ok as long it's big enough.Same is with rabbit cage.
Babies can live smaller enclosure but you should upgrade bigger one after few years.
I'm not that experienced myself,still learning everyday,so I hope that more experienced people will tell you more.
