I had to go through a similar setup at the start of last year - with the exception that i had to pay for it all myself, so I was working on a budget.
I got a kitset office - 2.4m x 2.4 meter with a small veranda/sheltered area out the front. This had to fit an existing concrete pad so it was perhaps a little smaller than I would have liked. (But then again, I didn't want a huge building taking up all the space in my back yard!)
I insulated and lined it myself. I was expecting it have issues keeping it warm in winter, but ended up with the exact opposite - with no ceiling space it's very hard to keep cool in summer. I bought an air con unit about a week into using it.
I got a sparky to wire the unit up to the house - mains only (no data). I had to dig the 1.5 meter trench myself. Internet is just via wireless into the house - speed is fine (about 35M) and no worries about redundancy. I use a laptop and have data on my phone so in the event of a blackout (which I wouldn't expect now days) I can just use the laptop and phone for access.
You'll definitely want carpet, I wouldn't bother with a plastic protector. White boards etc too. You want it feel office like imo.
Security - I got an alarm that texts my phone if ever there is a break-in. After a while though I stopped using it and often don't even bother locking the door at night. My laptop comes in to the house with me at night, and the most expensive thing left in there is my keyboard or maybe the air con unit. It's hard to make a space with glass windows/doors secure. The alarm will hopefully scare people off if you are home, but if you're out then they're kinda pointless.