Lots of advice on here and obviously some of it depends on you budget and climate/ground conditions. I would offer an alternative solution that may be a consideration depending on your priorities, look up a UK YouTube channel called Oakwood garden rooms and see how he does his rod foundation system (using generic supplies you should be able to get anywhere), very easy to do, very easy to level, will cope with almost any ground conditions but will be more expensive than some options, it’s at least worth watching to see how easy it is.