Converting a older cyclocross bike into a gravel bike can be a great way to get into gravel riding without the harsh price tag. Cyclocross racing bikes are essentially gravel bikes with one big exception. Cyclocross has always been happy with around 32mm tires when the new world of high performance wide tires allows gravel riders to use much bigger tires. So determining if your cyclocross to gravel conversion candidate is a good choice is finding out what the widest tires that can be used. That is not always an easy task. Previous owners may have never used anything over 32mm tires so they may not know what will work.
Do some google searches for your choice and see if you can find any posts from people who have used your candidate for a gravel conversion.
In this case my choice was the 2012 Kona Jake CX offered on ebay for $689. I found references that it might support at least a 700x40mm tire. It turns out it supports a 700x43 with room to spare. It makes a great cross to gravel conversion.