Concrete base for double garage / log cabin

I am installing a double garage at the back of the house in the garden and would appreciate some advice on the construction of a suitable base.

1. Base about 5.6m by 6m (about 18 x 20 feet)
2. Garage will be timber construction of 70mm logs
3. Land is sloping around 25cm over the 5.6m
4. Base to be built of soil with high clay content.

If you are able to give any advice on the following then please do:

1. Depth of hardcore
2. Depth of concrete
3. Is reinforcement mesh required for a large base like this?
4. Is a plastic membrane necessary for this depth of base to prevent damp?
5. Given the slope & clay soil, is there a risk of base slipping so requires additional foundations?

It is a large area and I am keen to prepare the ground myself but will have concrete delivered.

Many thanks.

I personally don't see the merits of slab bases unless it is for a shed.

I would plump for trench foundations and brickwork, coupled with an insulated, or at least a membrane protected slab.