Used bikes for bikepacking

Hello!! 👋 I really want to get into bikepacking but can’t afford the cost of a brand-new bike, so I’m hoping to find a used bike but don’t know what to look out for. I’m still fairly new to cycling but have absolutely fallen in love and am always looking forward to time on my bike. Since riding my first century, I’ve gotten the itch to do more (like a lot more mileage). Currently I have a Specialized Allez that I got during Covid and a fairly old Nishiki Olympic that I purchased off Facebook marketplace. I figured getting a used mountain bike might be a good start, or I would be willing to convert either of those bikes and get a new road bike since the Allez is a men’s frame and it’s always felt slightly awkward up top (it was the last bike available during Covid).

Any tips or suggestions are appreciated! Thank you 🙂