I’m really bad at trying to recommend a bike at a certain price point – but I’ll give you some general info from my POV on gravel bikes, in case it helps.
Currently – my gravel bike is a Norco Search Carbon Ultegra, and I love it. It has clearance for 38mm tires. And I use 35mm Schwalbe G-One tires on it. I use it to race gravel as often as I can – which is 3-4 times per year.
However, when I bought that bike I wasn’t buying it as dedicated gravel rig, which is how I use it now. I was buying it as my gravel/road bike and swapping wheels. Now I have a dedicated road bike, so I only use it for gravel racing and cold weather training outdoors.
If I was going to buy a hybrid gravel/road bike today, I would take an enormously hard look at Allied Alfa Allroad. I have heard nothing but great things about it.
If I was going to buy a dedicated gravel whip today, I would definitely buy some version of the Open U.P. – for a bunch of reasons…not the least of which is the tire versatility, and it takes 700 and 650B tires. Most of the gravel I race on is hard dirt roads with some loose on top of it. But I’ve heard that Kansas, Iowa and Minnesota are much looser, deeper and choppier – I would want the 650B for that. If your course has any sandy patches or singletrack…you’ll want more volume.
I have also heard really good things about the new Trek Checkpoint. And I’ve always been a little fascinated by the Cannondale Slate – another bike that takes 650B and has some front suspension.
I know all of these are outside of your specified price range, but if you look them up and decide what feature will be most important to you…you can find a used, or less expensive bike, with some of those qualities.
One more thing you might consider – for a 24 hour race, I might think about using a hardtail mountain bike.
This whole endeavor sounds brutal and unappealing to me