At one level any method will work, you are just removing the dull parts. However, some will give you more control and some will give you better finish than others. With belts (and wheel grinders) you need to be careful to not over heat the blade and to not go too far.
Many hobbyist knife sharpeners use 1" x 30" belt sanders for coarse work. Hock or Lee even show a homemade base for a HF sander in their sharpening book. It seems like more frequent / serious sharpeners prefer actual belt grinders (lower speed & more power, I think) with 2" x 72" being the common choice, even though it's much more expensive.
For my (very) limited use, I have the HF 1x30 sander but got some cheap metal belts. (Same abrasive, but different more suitable binder, I think.) I use it only for the really coarse reshaping work and shift to manual techniques to finish. It's real easy to screw up on the sander, but my slips have only left cosmetic blemishes that are OK for personal use.
As far as grit, I don't recall how the sandpaper scale matches with the waterstone grit scale. (Maybe Patrick or David will be along with more precise info.) For kitchen knives, 1000 to 6000 waterstone grit range seems most popular finish (to leave some "tooth".) Specialized knives, (e.g. Yanagibas for sushi), are often taken 8K-10K or beyond. I think any belt that gets you to an acceptable starting point for your finish work on stones is fine. Sounds like 150 worked for you.
ETA: I checked, I bought 80 to 320 grit Aluminum Oxide metal grinding belts. I don't remember which I used. I probably started at 80 and think I used one more before shifting to stones.