For that kind of work you won't need more than two channels, I can almost bet on that. At least I haven't needed and 90% of the time I use just one. There are cases in troubleshooting when it is beneficial to probe more than one point at the same time, so there is that. But two has been totally sufficient so far.
I can't say what you can get for 100€ as new, but I have some basic Owon scope and it is great for the price. If I remember correctly, I got it from some Amazon Prime Day deal for around 150€ couple of years ago. I think regularly they are a bit over 200€. It is very basic scope, so that 100€ sounds a bit tight to me.
You should also consider if you need scope at all. Have you used logic probe and found out that it isn't enough for your needs? To me it sounds that if you don't know what kind of equipment you actually need, you don't actually know whether you need one. Don't shell money on a scope unnecessarily, because it is pretty much the last tool you'll need. For example, I would first invest to better soldering/desoldering equipment and multimeter, because they are much more critical for successful repairs. Most of the repairs I do don't even benefit from oscilloscope. In fact, most of my oscilloscope use cases are with black screening 8-bit micros where I suspect multiple failed chips: they can be annoying to troubleshoot without scope, but the architecture is simple enough, that you can quite easily probe clock, reset, data and address lines and memory refresh signals and quickly resolve what may have failed and you don't need to go blindly changing chips.
Oscilloscopes also have a learning curve, and that isn't a shallow one. Not so much how to use the scope itself, it isn't much more complicated than multimeter, but you need to understand what you need to probe and how to interpret the readings. It is not a magic wand for troubleshooting and I consider myself pretty bad with it, although I've had it for quite some time now.
