You want to end up with a plywood surface that's 24", (actually 23 7/8" after a saw kerf) x 150" right? Does the seam fall on top of a support like a file cabinet?
If not, I would support it all the way undeneath with an additional 3/4" ply or 3/4" MDF. You can edge the front like you said, but I'd leave the ends off unless they are visible.
I would make a half lap seam so that each section laps over the other by 1". I would glue and assemble the top off site, finish it off site and make it structurally strong enough to carry it to the install without fracturing the seam by backing it with a section of 3/4" which can be permanent or unscrewed at the location. You could just back the entire length which will make it quite heavy, but that's the way I prefer.
You would like the joint to be invisible but because of the grain that won't happen, but match the grain as close as possible. Don't use a wild grain but try to find as straight a grain as possible when you pick out the ply.
If the ply were sitting in a rabbett for the front and rear edge trim, a backer may not be needed. You'll need a helper with a knowledge of lifting heavy panels, so carry the thing on edge or vertically
until it's going on the final surface.