I was going to build a 18", even bought the blade, but we were able to purchase, at the moment non working, homebuilt 24" horizontal feed one.
Some thoughts I had on the build.
I picked up a long hydraulic cylinder for an engine hoist from Princess Auto, a Canadian sort of Harbor Freight. This would give me an adjustable down feed. I planned on mounting the motor, a 1hp , inside the pivot to add weight. I think I picked up a piece of 8x4 square tube out of a bin for the arm. For the pivot, I would double bearing it, flange through the square tube and pillow blocks on the frame. The adjustment would be at the pillow blocks.
For the vise rails, I was going to use square cold rolled steel .
For a machined true bed for them ,I purchased machinist's V-blocks from BusyBee, a Canadian Grizzly . The imported small ones are cheap on sale.
For the vise bearings for the rails I was going to use cam followers, they are already mounted to a shaft.
For the arbor I told the wife I needed a metal lathe. My son and I went together on a new 10x22. When he moved out it was "I'll buy you out" "No, Dad I'll buy you out" I ended up buying a used 12x24. I still haven't made the arbor.
My motto has always been if you can't make it right, at least make it adjustable. I planned on using a lot of square headed set screws(pusher) and shim stock. When I was young I could plan it out in my head and build it, doing the engineering as I went.
I hope this gives you some ideas. Good luck.