Jim,
Really, all in all, a No.1 is a No.1. The same basic action, different barrel with its accessories, and the forearm to match said barrel. With that said, preowned no.1's are pretty prevalent now, and priced at about half of new factory price.
The 243 Winchester case is a 0.473" head for the ejector/extractor fitment, so a 22/250, 243, 6mm, 25-06, 270 etc, etc all would work for the 243 as is.
If one ran across a no.1 in another smaller or larger caliber, the factory ejector/extractor is less than 10 bucks and is easily changed out. Preowned forearms and barrels show up on the gunsites, although most are hunting style rather than varmint style barrels.
If you can stand the No.1v configuration, then a factory taper is available from most barrel makers(in any length really), or one could consider a 5.5 taper Shilen will fit pretty close in the No.1V forearm already, and you have the option of a 26" barrel length. Only 26" in the factory 1v is/was the 220 swift, the others as far as I know were 24".
I have learned to do my own work in rebarreling, so I change barrels according to my whim of the day, I work cheap for me, so my time is not even figured in the cost................never has, too much fun doing the work really. One of the true beauties about the no.1 is its flexibility as to caliber and cosmetics.
A named maker stainless 243 bore blank is around 200-220 dollars, while the select match are about 20-30 more. Most smiths that do no.1's will rebarrel for 200-250, which may or may not include the drill/tap of the barrel for bases. As you know, the bases and ejector/extractor machining has to be timed, thus extra costs with some smiths.
If you decide to peruse the used No.1's, just keep in mind the forearm is probably key(barrel channel to some), as well as basic condition of the action/wood. Barrels can be changed at a flip of the wrist for those with the equipment to do so. I am more simplistic on my No.1's, change the barrel out to a good aftermarket barrel maker, make it fit the forearm, and it is good to go. If you ever clean a custom aftermarket stainless barrel compared to a factory one, you will see the ease at which they clean up.
Looks like perhaps you have a 1v in 25/06 now that might be considered for update to 243, any gunsmith worth his salt can do that in his sleep. You might price that out locally there just to see how it would go for you.
FWIW