I think big question is whether you want to DIY something or if you want a more commercial, ready to use out-of-the-box solution.

The open-frame Cartesian kits (which are basically the same hardware you'd use for a CNC router) are the cheapest for their bed size, and you can make them quite large. I've seen some Youtube people with what look like 4ft x 6ft machines. But the common designs seem to just leave the CO2 laser tube just sitting exposed... no case or anything. And that strikes me as not safe unless you have a dedicated workshop you can absolutely close off from any wandering people / pets / whatever. And you'd need laser safety goggles, obviously.

The K40s seem to be a good value for the money but there are a lot of horror stories around of people getting units that need a lot of work before they were able to run anything. At least, that seemed like the case a few years ago. If you don't want to mess with that, there are US-based companies that seem to be buying the components from China and assembling them (or at least QAing them) here in the US before shipping them to customers. You pay for this, obviously, but it means you aren't dealing with someone in China and begging them to send you a new laser tube if yours arrives busted or something. Boss Laser and OMTech are two brands I know of... although Boss has/had a mixed reputation back on Reddit's /r/lasercutting and not sure if they have changed.

Right now, OMTech is selling a 100W 20"x28" (presumably 500mm x 750mm) unit that looks exactly like my generic Chinese machine for $3800 USD. That's only $800 more than I paid for mine used, locally... and it has a 2 year warranty. Not bad IMO.

If that's too much, you can drop in power down to a 60W or 40W unit, although after using a 60-70W unit at TechShop a few years ago I decided I didn't want less than a 100W one; it just seems to give you a lot more options especially for cutting and vector engraving.

And of course if you have money to burn (or someone else's money) then you can get a "real" prototyping laser like an Epilog... they are sweet machines but dear lord you pay for it.