Be careful. Single 24x60" sheets of 3/4" MDF are pretty easy to lift. A doubled sheet is manageable. The countertop - 24x72" panel of 1-1/2" oak - weighs something over 100 pounds. It takes real care to lift safely. The joined top - 3" thick of oak and MDF - is past the range that can be lifted safely by one person. Don't try. Get a friend to help, or rig a block-and-tackle.
Not sure about Midwest but I have a 24 x 14 Game rm attached to my shop and I put a small window unit/heat pump in the wall. It stays comfortable all year round. It get really hot here and we had a several day in the 20's this winter but it's insulated well. That might not be as cold or as hot as in some places but the unit I haze does not run that often and has a low cost rating. I'm planning on putting a larger unit in the shop area even though I do like to work with the garage door open. The door is insulated so I can run A/c in there. Just my .02.
The shelf in the first picture is made of red oak plywood. You can choose the wood type, color and design as you like for your project. In case if you need more help understanding this project, you can refer the source link below. It discusses various items used, steps and tips and personal experience of the author who personally built a Zigzag shelf.
The Cape Ann Shed was designed with coastal New England in mind. So yard work may feel like you’re actually on vacation. From Cape Cod to the beaches of Maine, you will find a Post Woodworking Cape Ann shed adorning the yards of thousands of homes. It’s a popular style with our customers – whether they make their home by street or by sea. An affordable choice for an outdoor structure, the Cape Ann brings functional elements with nautical appeal.