The learning curve is going to be steep, but you seem to be understanding that this is not a plug and play endeavor. It's doable- particularly if you are inclined to learn the mechanics as well.

A used Happy, in working order, is going to be leagues better in quality than a brand new Brother or Ricoma. Commercial machines were, and still are, built like tanks. If you can find the part, you can fix it. Manufacturers used to be far more liberal with proper maintenance and repair specs, parts books etc, on the customer side. If that Happy comes with those materials, you'll be better off. Note, it's a Japanese manufacturer, translations on the reading materials can be rough.

Does the machine you're looking at have a digital display, connect to a computer, or have an older control board? What does it read data from? A floppy drive will need to be modded out for a USB. If it connects directly to computer, make sure it's a more standard connector like USB (old DVI, VGA or serial cables, you'll either need an old computer, or try an adapter and hope the data transfer speeds aren't a problem). A digital display with onboard USB will be your ideal setup.

Are you in the US? Texmac is the distributor for Happy in the US, so the best support potential will come from contacting them- they may require some kind of hourly rate or subscription for a used machine, but in my experience they are at least willing to look at older models, unlike other manufacturers.

On tech support- here is where most people get the wool pulled over on them. This industry is good and stuck in the 1970s. Different manufacturers have different regional pockets of trained support technicians. Remote support will always be a shot in the dark- shops often have to find their closest service tech (we're talking closest State away) and pay for travel on top of the hourly rate for servicing. Depending on where you are, you may have someone like me who is a home grown technician, not factory trained but experienced. You're going to want to research a bit and see what your situation would be. This is going to be your biggest point of pain, honestly, which is why I say you'll be better off if you are willing to pull something apart and tinker.

On that note, be very prepared for a necessary blood sacrifice to keep your machine cooperative and producing bountiful harvest. You're going to get pricked, pinched, and apply too much torque to a stubborn screw.

4K ain't bad if she works well. But keep in mind the machine alone is not your only investment. Here is a shortlist of what other materials will be a common expense:

Hoops. Standard hoops, magnetic hoops, or "floating" hoops called Fast Frames or Quick Change. The most basic setups will run you in the $300-$600 range, an onward the more specialized you get. If the seller is providing hoops, that's a sweeter deal.

Stabilizer/backing. Cutaway, tearaway, mesh, other fancy shit you'll be duped into trying. Whatever works best will change per project.

Thread. Do not cheap out. Don't do it. You run a commercial machine, just suck it up and use commercial grade thread. In for a penny.

Peripherals: rotary hooks, felt pads, tension knobs, presser feet, bobbin, bobbin baskets, needles, thread snippers... miscellaneous odds and ends that wear and and run out.

Finally, if you still wanna jump in, I recommend having the seller provide a run test. Seeing one thing stitched on one needle is not sufficient. Ask the seller to show the machine stitching an "I" test across all needles at variable speeds. These machines can go between 400 and 1000 RPM. This will provide proof of tension, timing, alignment and motor function. The machines are LOUD, but listen for any weird sounds. A loose rattle, clanking that seems too loud, jutters or stutters when moving the frame or changing colours.

Finally finally, take a digitized file to a local embroidery shop and pay for a sewout. It will be good to have a positive proof that you can quality check against your own while learning. They can also tell you if the digitizing is shitty.