5160 is a great choice for an impact blade.
The think about a khuk us that they tend to fall one way or the other. A thinner one will take on vegetation well but won't chunk out wood as quickly. A thicker one will take down a 6" tree in no time but will lay reedy stuff over instead of cutting it.
For a 18"-ish overall length, a lot of what you want to do/carry will need to be decided. When I was a younger fella just getting into these knives 20 years ago, I would take the biggest/heaviest I could get. Now, I tend to leave the big stuff at home or at the base camp and just pack a 12" overall smaller one for odds and ends chores. I may opt for one of 1/4" spine 15" villager models if I want more cutting performance yet want to keep weight down.
Of the length you are describing, my favorite user that I am willing to pack is an M-43 Lightweight model from Himalayan Imports. The spine on this one comes in at 5/16 which is a touch under the normal HI overbuilt 3/8. I haven't weighed it, but I would guess it comes in around 22oz which isn't bad for an 18" knife. The old adage is that a fighting khuk should come in right at or under an oz per inch. 18" bruisers easily clock in at 32+oz and hit like a ton of bricks. Then again, I have a dedicated 18" fighting khuk with a sharpened and hardened clip that easily comes in around 28oz. However, it's not made for dazzling flourishes. It's basically chop and if you miss, snap a back cut and hope your do enough damage to end the fight...but I digress.
If it were me, I would try 5160 and keep it fairly light. It will offer decent performance without wearing you out. A lot of it will depend on who is making it and what methods they are using. Modern steels using modern methods can yield a stout but thinner knife that will still take a lot of abuse. Traditional methods have different drawbacks.
Like all tools ingrained within a culture, there is a learning curve to using a khuk so that you don't hurt yourself or your knife. They truly do take mere minutes to learn and a lifetime to master. Watching a Nepali use one knife to chop wood and prep dinner all within minutes is amazing to watch. These are generally thinner stock blades glued into a wooden grip. They would be pretty easy to break if misused, and one of the reasons you see so many overbuilt tanks in the khuk world.
Good luck
