Really great points. Upgrading components later is a great strategy. My husband is a cycler, & worked in a shop for a while. The only thing I would add, is you should get fitted (or sized for lack of a better term) for a bike. Even if you buy used, you can look for something n the size range. This is much more important than I first realized. Go to a professional cycle shop if at all possible & ask about this.

Getting a frame sized for your body & learning where the seat should be, (moved forward or back as well) will mean the difference between a comfortable, enjoyable ride & discomfort in shoulders, arms, low back, &even your hands.