I'm a brand new member and haven't started a hive yet. But I've been doing a lot of research. I have a relatively small space (so, backyard beekeeper) in a wooded setting in central Wisconsin (zone 5b) (see my profile pic and attached - the hive will be placed near the bushes by the lake). I've decided on a 4' top bar hive, which I will build myself based on Philip Chandler's Youtube instructions. I also hope to start it by trapping a local feral swarm. We'll see. Clearly, I'll need guidance.
I don't see much wrong with your plan. Although I would suggest building a top bar hive with a 19in top bar, simply to maintain compatibility with langstroth equipment in a pinch. As well as it creates a deeper larger comb, which in theory should be better for wintering.
Top bar hives are great for learning how bees work. Very low input cost, fun to work. Have really enjoyed mine.
If you find that top bar hives simply won't work in your location. Give the Layens hive a whirl. You can build the frames to where the top bars touch just like a top bar hive. Some people like that. Some people don't. I personally prefer them touching.
As far as trapping bees. Go ahead. Sure you might just be catching swarms from some other beekeeper, but this is still way better than ordering bees from Georgia like that other beekeeper who lost that swarm did. Partly because you aren't spending $$$ for those almond bees that are worthless up north.
Look at it this way. Your cheap hive and free bees are still going to give you valuable experience. If they don't survive, you really have lost nothing. Later on when you feel more confident, buy northern stock.
I certainly wish more beekeepers would start like this. Me and most other backyard beekeepers started with a langstroth hive and a 3lb package of bees from Georgia asap in the spring. Of course, just like the book said....