DIY

My first house was a rental for 12 years before I bought it, and it sounds like it was very much what you're describing. I don't want to give you advice, necessarily, but I can offer my experiences and perspective.

A lot of what you'll be tempted to do is to "make better" and "correct" what someone else has done before you. It's tempting to just try and "fix" this sort of thing. I tried that route, and found that I spent far more time, money, and emotional labor doing that than if I just ripped out the whole thing, and started fresh. Once I started fresh, I was no longer reverse engineering a clusterfuck. That part seemed to be the most difficult.

It's hard to convince yourself that starting fresh is less work and often less money for better results, so it's totally understandable that you may have to teach yourself that lesson by trying to fix bad work.

Secondly, check out some of these YouTube channels where I learned a SIGNIFICANT amount about how to do things, or what to look for when I felt like I needed to hire someone.

Scott Brown carpentry: Construction, renovation, and a great attitude every Friday

Samurai Carpenter: Look for the videos about 4-5 years ago when he was renovating his house

Tile Coach: bathrooms/showers

Home Renovision DIY: he's got a video for damned near everything

Finish Carpentry TV: (older episodes) for trim, doors, general finish carpentry

Insider carpentry: Also finish carpentry, often tailored to the business side of things or focusing on production environments. However, tons of tips for regular folks

Life uncontained: general house diy construction tasks

Frank Howarth: Look for videos about 4-6 years ago about his house renovations

Essential Craftsman: specifically the spec house build playlist

Vancouver Carpenter: he's good with drywall stuff (so I hear)

Additionally, anything else you want to do, just search YouTube. Watch a few different videos to get a feel for the parts of the task that everyone does the same way. You can also watch a few folks do it, then decide whether or not you think you're capable.

If you're concerned about budget and tools, check Craigslist for both used tools and building materials. If you're ok with buying new tools, it's hard to go wrong with the Big 3: DeWalt, Makita, Milwaukee. Really, it just depends on what your favorite color is. Ultimately, you're committing to a battery style for all of your followup tool purchases, so just keep that in mind.

Full disclosure: my wife and I re-did every room in that house, which is why when we moved, we built new because we absolutely did want to do that again. It's WORK. It's satisfying and rewarding work that gives you much more confidence in yourself, however.

Good luck, and feel free to PM me if you have questions