Start by figuring out whether anybody will be inspecting, and also whether or not you have any reason to have "exotic" foundation requirements, such as extreme freezing, how deep is the frost line, earthquakes, whatever. What is the local authority going to look at. Around here, you could ask any builder what sort of foundation they'd put under a building like that. That used to be a very simple question, and whether it's simple or not depends on the degree of scrutiny and your location.
Requirements are going to be 100% local and I think you'll want to flee the internet and walk into your local permit office and talk to the people in there.
There is no practical reason for engineered blueprints for what is essentially a farm building, and it's not really normal to make that a requirement. if you're building it yourself, and you're a reasonable person, then you can obviously tell yourself what to do. So "best" might mean different things here. Architects are for making buildings pretty, so that is a very good idea if you want a pretty building. Some of the shops here are very attractive and some are industrial-looking and some look like farm buildings. It varies.
Plumbing, I think, is fun to do and super-easy, so I can't really tell you how much help you need there. The main thing is to decide what you want installed and where, and then the plumbing just results very naturally from that. If you can't figure that out then I would think an ordinary plumber would be wildly overqualified to advise you.