It really depends on what you want to do with it. If you just want to ride fire roads and mellow trails then anything will work. If you want to increase speed and ride more technical stuff then you have to have to be a bit more selective.

Gravel bikes, as they are marketed now, are road bikes with off road capabilities. Their appeal comes from the ability to ride mixed surfaces, reasonably fast on the road and reasonably good in the dirt.

Some people are quite fast on the dirt with gravel bikes but when things get sandy or rocky a mountain bike is generally better. I've done some rides with faster people who were on high end gravel bikes while I was on my full suspension mtb. They are so much faster climbing dirt roads than a heavy mtb. The high end ones tend to have 38mm tires but some have 650 wheels and 44s.

I've owned a few old hardtail mountain bikes and some are so crappy I wouldn't want them for anything. I have thought about getting one for gravel-type rides but new gravel bikes can be had cheap. Cannondale starts around $1300. Topstone 4 | Gravel Bikes | Cannondale

You can get drop bars with a huge flare on the drops. Here is a friend about to drop me with his 20ish pound gravel bike while I'm on a 32 pound enduro bike. You can see the flare in the bars.

Same bike.

My friend's bike with 650 wheels.