Hi Anthony and welcome to the forum
The deeper you can have the substrate the better, but it doesn't have to be all the same depth all over the table as Horsfields love to dig
Your table sounds a reasonable size for your tort.
yeah your tort will be fine on his own, Tortoises are naturally solitary animals. Though there are loads of members on here who have more than one, and you will find when you have one, you'll want more
, so maybe better to get two together from the same breeder, as otherwise you will have to quaratine them for 18months before putting them together.
Not sure how young you will be able to get a tort, as it is breeders individual views on when they decide to sell them
A few members on here use rabbit cages, so yes one would be ok.
Hope that helps