I have and use 9x12 folders. I would guess you might be able to adapt 4x5 film holders, but I don't think it would be a lot of fun. But you've got me thinking.
You do want to look for one that comes with cut film holders that fit or be sure you can get cut film holders. A 6x9 back is also a good investment. I have a couple and use them on occasion. Getting film holders or 6x9 backs can be a challenge. Sometimes you just have to buy and hope. If they are being sold separately, at least be sure they have the film sheaths. Film sheaths by themselves are even harder to procure than the backs. Backs also have different ways of loading the film. You may have to try a little to find out which way any one works. It's loads of fun when you have two or three different ones to load at the same time. 😀 Light tightness may be a problem as well, so sometimes the felt will need to be replaced.
Some 9x12 are heavier than others. I understand the Patent Etui is very light. It looks like it would be. VAGs are heavier, Zecas are not as heavy as VAGs. Any with a RF are going to be heavier. They are also going to be more expensive. The RF may or may not work. I have some on which they don't, and I haven't had the time to work on them. Zone focusing usually does, and certainly ground glass focusing works (make sure any you buy have a good ground glass).
Also ensure the GG back closes and latches, or pay less. Make sure any you buy have a working infinity stop. Ensure the front standard is tight and that the front opens, closes, and latches properly. Ensure it has a brightline finder, and a wire finder on the front, with its rear portion on the camera body. Many times these have been taken off for parts. Be sure the wire finder isn't broken. You may have to buy an adapter for the tripod mount. Many are the larger, not the 1/4 inch. Adapters are only about 3 to 5 dollars from Freestyle and others.
Most have rise and fall on the front standard. Many have shift. A very few have tilt. Fewer have interchangable lenses. I have one in 9x12 and one in 6x9. I have yet to find a lens other than the normal for their format.
Normally you can find aux lenses for telephoto and wide angle. They will no doubt reduce resolution a little, but shouldn't a lot, and you do have LF in the 9x12. You can also get filters for them.
All in all, I like the 9x12 format. There are some films you can buy, and you can cut down any 4x5 film available for use in the 9x12 holder. They are smaller than 4x5 RFs, and lighter, except for the cut film holders, Metal holders do add weight quickly. 3 to 5 working holders will probably do you, especially if you get a roll film back. I have over 20, but would never carry them all unless going on an extended trip. That might be handier than keeping track of what ASA sheet film I had unloaded with a changing bag.
You can develop in a tray, just like 4x5, and as you have read above, other methods work as well. Printing will require a 9x12 holder, or a home-made insert for a 4x5 holder. The same for scanning. For my Epson, I acquired some heavy plastic about the thickness of a 4x5 sheet, and cut to fit the 4x5 holder. Cluszy, but workable.
Hope that answers some of your questions.
EDIT: I forgot to mention shutters and lenses. Lenses are usually easy to clean if they are just dirty, not scratched. Shutters will often need cleaning. I haven't tried the Compur shutters rim set yet, but will some day. The dial set seem easier to clean. Ibsor are very easy to clean with a little care. Some day I intend to put up a thread on cleaing them. If there are bad springs, as long as you have the original, even if broken, you can probably get a good replacement. Mostly, just a good cleaning in naptha will bring them back to life. The Ibsor will need a little (very little) grease after the naptha bath.
Also, if you buy off the 'net, make sure the seller will stand behind his description, which must include the good state of the bellows.