I was a subcontractor for Basement Experts / Owens Corning for years. Worked my but off for them. Just a warning for those subs out there considering working for them, in the beginning they pull money from your checks as an insurance policy to make sure you guarantee your work for a year. I can understand that. But once the year passes with no call backs they are to return the money. Its been 2 years and I still haven't seen my $700.00. Beware subs, they'll screw you the same way they do everyone else.

It seems you have made the decision to update the space. I totally understand your desire to do the update over time on your schedule. DIY'ing everything? Here would be my probable event sequence. Sounds like your basement was dug before a requirement for egress windows so a grandfather clause should prevent your having to install one provided, as you say, this isn't living space. Develop a scale drawing for everything in the finished space. Include walls, lights/outlets/switches and any features to build around (stairs / mechanicals) or added (Powder Room/Bar etc.) Place masking tape on the floor for wall location(s), can lights and anything else planned. This will force you to completely think through the build, develop a materials list and enable you to begin shopping even before beginning the project. I'd include a back-up sump pump as further protection for the investment. Remove the carpet. Remove the existing drywall and insulation. Now you can inspect the existing studs and the construction quality. Possibly you can leave many in place. Kill any mold and replace any infected studs. Now is when you need to think about permitting. Know the requirements! Talk with them, submit the plan for review and approval. You'll probably need plumbing/electrical/insulation and final inspections. HVAC? If you have forced air at minimum install a cold air return. Generally minimal heat will be necessary for a basement except seasonally (spring/fall) when auxiliary may be necessary. I installed an electric fireplace to accomplish this. Any plumbing required? You don't mention the need. Proceed with electrical. Inspection upon completion. Have expansive foam insulation installed professionally, then have the insulation inspection. Drywall. Not familiar with Thermex so can't advise. Final inspection. Recarpet or tile the floors. Trim the doors/walls. I'm sure this is a simplistic overview absent more specific facts. Good luck.

We were going to put a media room in our basement but then decided against it as we would probably never use it...it's just so darn convenient to hang out by the kitchen. So we used our lower level for my craft room, an exercise room, a guest room and a full bath plus a store room. All of these things are things we could live without if we had to so we could live on one level should lifestyle require it. However, if we had kids, making a kid space would probably be the best thing to do with the space as, as kids grow, they want their own space away from everyone and everything and basements are perfect for that. (Yet parents like their kids home so they know where they are and what they're up to.) So I'd suggest building a large space that is flexible for multiple purposes as the needs and wants of the kids will change as they grow....

I like that space! We recently painted the bar area in the lower level of our house. It's basically a run of cabinets on one wall, with a sink in the middle and no appliances. We went with white, and now I wish we had done something else - the white feels too much like a regular kitchen vs a fun bar area - although it's technically not a bar and we don't use it as a bar. Here is a blue basement bar on a blog I read occasionally: https://www.thelilypadcottage.com/2018/03/basement-wet-bar.html