Well, that sounds like a shop I'd love, but on a $30K budget, you may be hard pressed.
I'll be building a shop in the next 5 years. Being MUCH further north than you, I'm focusing on insulation as well. I'll be using ICF (concrete) blocks for the walls and include radiant heating in the floor. I will only heat my shop to 5-10C (40-50F) and use a wood stove to bring it up the rest when I'm in there. I'm also looking into adding what would look like a boxed chimney from the outside but is open and extends from the ceiling with a skylight at the top. This will allow natural light all year long and by opening the skylight in the summer during the day, provide natural cooling as the hot air will rise. I will also use large windows on the south wall and have a properly sized over hang above them. This will provide all the heating during sunny winter days but not in the summer when the sun is at a much stepper angle. There are plenty of online tools to calculate the overhang size as well as the window height. I'll also plant leafy trees on the south side to provide shade and cooling in the summer but not in the winter. By doing a bunch of stuff that is very low cost, I'm hoping to have a nice heated shop that's cool in the summer and doesn't cost much to operate.
The problem with ICF block construction is that you must finish the interior walls right away as it's a hazard in the event of a fire. That is building code here, not sure about elsewhere.
Adding a bathroom may be $30K on it's own if you have to add a septic system. A holding take works, but costs a lot to constantly pump it. Even if you can connect to the existing septic or sewer line, extra walls, finished floor and a basic bathroom (with shower?) is going to bite a few thousand out of your already very limited (for that size) budget.