I would not replace the table saw with a cnc. I have both and will keep both. Unless space is in such a short supply, like say a tool room on a ship, I wouldn't consider it. They aren't the same even though there are a few things that they overlap on.
I have a vertical panel saw that I use but not all that often as I do end up nesting parts on sheets. It is just easier all around if I am doing more than a few cuts. I do use it when I need the narrow saw kerf or it is something relatively simple. Most of the time though, either the drawing file is provided with all the parts already nested, or I am drawing up the whole project anyway so it is really easy to just nest the purely saw parts in with all the rest while I am at it. I tend to get very good yield this way also. But, I am not going to use my router to rip lumber or all the other misc stuff I do on my table saw.
As for cutters, yes there are plenty of options for cutters to leave a very nice finish on the top and bottom as well as the edges. For parts I want both top and bottom nice I use a compression bit. Some call them and up/down bits. They are made in such a way that the top portion of the bit has the flutes pushing the chips down while the bottom is pulling the chips up, hence the name compression.
These are really good for full depth cutting. If you want to do a dado or rabbet, you would want a downshear bit, it pushes the chips down only, so it is great for partial depth work where you want the top face perfect, but it will blow out the bottom if you try to go all the way through. There are also upshear bits that are just the opposite, they pull the chips up, so they are best for evacuating chips and will leave a nice face on bottom, but can chip or blow out the topside. They can also physically lift the part off the table if not held well and you push it too hard. Each has it's pros and cons and I would suggest having all 3 on hand.
There are also roughers in both up and down shear and I have seen some plywood specific compression bits too. Roughers are awesome, they look similar to a metal working rougher endmill and leave a grooved edge, but they really do hog the material away and leave the finisher for just finishing. They also last a long time. I set up a shop almost 2 years ago running a 1/2" dia rougher in solid wood cutting guitar bodies running all operations at a min of 50/week and I have yet to need to get it resharpened. It did cost $180, but it reduced the cycle time by half on a few programs.
As for the type of machine I'd suggest; I'd have 4x8ft and a toolchanger minimum. From what I have seen, the guys that get small tables are either not serious and or are making things like guitars only. A person can do good work with a half sheet machine, but it is like everything else, you will find you regularly need or want to cut something 49" + long etc. Also, the difference is literally only 4 more feet, you still have all the same width and accessories with a 4x4 machine without near the capacity. If you have the option a vacuum table is also well worth the money.
For the toolchanger, if you get a machine that you have to change tools manually, it adds enough time to it, you will honestly find yourself going back to old methods. Think about cutting a cabinet part with shelf/hinge holes, a dado for a back and perim cut. That is likely 3-4 tools needed assuming you route the hinge cup vs drilling it.
You can do this a couple ways, either change tools for each part individually as you run it, or setup so you can run one tool on all the parts, then switch tools and run them all again and so on.
I used the cabinet part as an example, but it is similar on solid wood parts too. I use a rougher any time I can as the tool pressure is very low so my workholding can get sketchy without much consequence and it is much faster cutting. So even if I am only blanking out a part for radius molding that gets finished on a shaper I use 2 cutters.
Grain direction; if solid wood, I still use standard methods of cross grain first, if sheet work I mostly just cut the full perimeter at one go without concern for grain direction. If the veneer proves to be a problem, then cutting sequence can easily be changed to fix it.
As to new or used machines, I think the biggest reason to buy new is to get the training with it and you can stipulate a turn key package before you sign off on it. There are alot of things up front to decide on and learn before you get to the machine and the better companies will have the path from start to finish laid out for you. Of course you do pay for it, so the price is high. If you can figure things out on your own or with help from forum friends etc, used is a really good way to make it happen.
Just a FYI in case you don't aleady know-
CNC basic work flow: You need to draw the thing on the computer (CAD software), then you need to program it, which means putting toolpaths on the drawing you made (CAM software), then you need to run the toolpath program through a post processor. The post is just a machine specific program that takes generic toolpath info and translates it into actual code that your cnc can read. This is a specific program that only works with your CAM software and your cnc machine. You can get one from someone else if your combo is popular enough, but most times it needs to be written for you and depending on who you get the software and machine from it can be free or cost alot. Once the code is posted you can then run it on your cnc.
Depending on which path you go down, some of these steps are obviously separate, or they can be rolled into what acts and looks like 1 or 2 steps, either way all these steps are happening to get the cnc to go.
I know it can be overwhelming but figured you should at least have an idea as to what to research and be aware of. Skipping a step like the post processor can turn into a big hurdle and cause some long delays if you don't know to ask about it up front. Feel free to contact me if you want to ask any questions.
