Billhooks can be great tools, if you do buy a new one be prepaired to spend some time reprofiling the grinds, they tend to be pretty rudimentary & barely sharp. You may also need to straiten out the edge somewhat. A convex grind is also preferable for durability & a bit more penetration.
Old hooks will of course vary alot in condition but good ones will need minimal work if any to get the best from them.
Here are some I'm slowly working on...
Top - makers mark indecernable - Newton (Newtown) - rehandled in Yew by me.
Left to right -
Ellwell 3902.7 Spar hook - awaiting rehandling
Harrison Bros. Sheffield small full tang - awaiting rehandling - Any ideas what this pattern is called?
2 Morris of Dunsford - Saffordshire pattern (awaiting reprofiling) & Newton (Newtown)
Both my Morris hooks have a slight distal taper though a Dave has pointed out nothing like the Ellwell & most older hooks making them a little heavy for prelonged use - you'll want to take the varnish off the handle too.
There is a wealth of information about billhooks here on the forum all of which will help inform your choice - or muddy the waters
I'd suggest trying to find somewhere 'real world' selling them - country/craft/game fair, old tool shop etc. That way you can handle a few & see what feels right for you.