I don't see the problem with a used bike. This isn't like a car where there are thousands of parts that can break. I purchased a 15 year old cannondale that I still have today (4 years later) and I have never had to replace a single part. A used higher end bike is worth more to you than a cheaper bike. You said you want a mountain bike without suspension because you will be riding mostly gravel trails a cyclocross bike might be a good bet for that.