For the type of riding you describe, "light or crushed gravel and a bit of tarmac," 32mm to 35mm tires should be plenty wide. Put a pair on a bike you already have and see what you think.
Here's a seventies Motobecane Le Champion I outfitted with brifters, clipless pedals, and lower gearing along with the wider tires for exactly this sort of riding.
Brent