If you've watched many of the top shooters in CAS, a majority of them use an 1873 rifle. Essentially the rifles are made by Uberti (Italian manufacturer) and imported by EMF, Navy Arms, Taylors & Co, and a few others. The toggle-link action makes it a very fast rifle since the round moves up in a horizontal motion rather than a tilted maneuver like a Marlin. Just like the Italian SA's, the fit/finish/quality on all of the importers can vary. Examine the gun and see if you like it for fit and finish before you buy it.
There's a few diffrent styles of '73s that you can buy on the market. A 20" carbine saddle-ring version, a 20" short-rifle with straight-stock, a 28" long rifle with straight-stock, and then my personal favorite a 20" deluxe Border rifle. I own both a 20" short-rifle and a 20" deluxe Border rifle from EMF, the latter is my favorite of the two. Why the differences? Just like cars, everyone has different tastes. A few of my friends love the longer rifle version. Personally, it's just a little too long for me. The deluxe Border rifle has a curved pistol-gripped stock instead of a straight angle from the trigger to the butt. I find the angle of the Border rifle works better for me. The short carbine is similar to the short rifle, except it comes with a different rear sight and has a saddle ring on the left receiver side of the rifle which most people remove if they are shooting it for CAS. 73's are pricey, with the carbine selling for $800+ and the deluxe Border rifle fetching around $1k. Calibers can be found from 32-20 all the way up to 45LC. My short rifle is in .357, my deluxe Border rifle is in .38-40 for blackpowder.
If you're going to spend any money on an action job, do it to your rifle first. You'll find that it will do the most good for you right away in your shooting scores. Action jobs are a nice to have option for '73s. An action job smoothens out the rough edges in the receiver, cleans up the metal on the carrier, and basically makes it a smoother operating gun. Most 73's right out of the box don't need much to make it a smooth gun. However, you can push the envelope and make it smooth and have a shorter lever throw by installing a "short stroke" action kit.
Some shooters (myself included) have their rifle lever travel shortened. Why do this? Removing an inch or so of travel makes the rifle faster to cycle, giving you faster times and higher up on the match award food chain. If you're a gamer or you really want a really smooth gun, this is yet another mod you need to have.
If you want to do it yourself and have good skills in the gunsmithing area, then the short stroke kits from Cowboys & Indians is what I'd recommend getting. They've got the shortened toggle kit and the shortened carrier lifter arm which makes the whole thing run. Most of the time it's a drop-in replacement, however there are times when some grinding and fixing on the innerds are required to make it work. If you're not comfortable doing it, then please find a qualified gunsmith in the CAS circles that know how to do it. The upside of the short stroke kit removes about an inch or so of travel.
So who's the best for action jobs? It's all opinions but I've had Squibber & Sheriff Bill Murphy do the action jobs & short-stroke mods on my '73s. Both are darn knowledgeable on what to do to a '73 to make it sing and run really smooth. Both guys have been doing this stuff for a while and have many happy customers. Sheriff Bill did the job on my Border rifle which runs really smooth. There are others out there in the CAS world that do action jobs but I'd rather hand my work to these guys first since they are closer to me, I know them, and they have very good reputations of doing quality work.