I would buy a used 12” + jointer and go HSS or Tersa style if the machine already had it.
Have you ever used a jointer with a helical cutterhead? I don’t understand why you’d want or need this on a jointer as a milling machine. Planer, sure I can see the appeal. Often times the helical knives leave a slightly scalloped surface which seems like it would be terrible for edge jointing for edge glue ups and need an extra step to get rid of the scallops in prep for a tight glue joint.
I have a 12” Oliver with Tersa style indexed cutterhead and can go straight from the machine to edge glue ups with very very clean edges.
I used to own a nice 8” invicta DJ-20 and have also used a DJ-15 with Shelix for edge jointing and I’ll take the 12” + with straight knives everyday. I paid 1/3 to 1/2 the price for a new Powermatic 8” Helical machine for my used 12” with Tersa head about 4 years ago. Deals are out there on used machines; I see them frequently. You just have to know where to look, look everyday and be ready to pounce when the right machine/price/location pops up.
You don’t need a new jointer or a helical head, just a good one and long beds, mass and wider cutterhead is really what makes a difference on a jointer, IMO. That, and fence that holds 90*.