Honestly, I'd rethink the idea of going fully rigid if a lot of the miles are going to be on gravel towpath.

If this path is anything like the ones I've ridden, there will be places where it just sucks. I was riding my Bad Boy (rigid fork) on one here and took it to the adjacent paved trail at the first opportunity -- I got tired of having my wrists & butt banged up. There were parts that had muddied and hardened into clumpy patches, and there were long stretches of washboard. I could do one of two things -- stand up the whole time or find another route.

I'm probably one of the last people who'd still recommend a suspension seatpost, too. Good for smoothing out annoying bumps without the weight, complexity, and inefficiency of rear suspension.

An ideal setup would have a lockout fork so that you can ride "rigid" when you need to, but that'll bump the price by a good chunk.