Around here, the conditioner types of mowers have fallen out of favor.
In 1980 we had someone mow our field with a self propelled mower/conditioner, but since then, the mowers have always been tractor implements.
The Disc mowers apparently eat up a lot more HP than the old sickle bar mowers, so depending on the size you're interested in, 30 HP may be a little light. unless you have a second engine on your mower.
For my Kuhn disc mower, the cutter bar is more complex than you might think. Each disc rotates in the opposite direction. And, the discs are all geared so they can be synchronized, so the long part passes the narrow part on the nearby cutter.
You may choose to rebuild a cutter bar, but I doubt you will want to make one from scratch in your home shop.
If you just want the hay piled up a little bit, but not conditioned, you may look at the drum mowers. They would be a very simple design that will automatically windrow the grass to some extent. And, if you are wanting to build one from scratch, there are a lot fewer parts than a disc mower.
I like the photos of the mower/combine, although there are certainly other "options" if one wants a wide mower. A couple of brands make front mount mowers, as well as wing mount mowers.
See:
Krone North America -*EasyCut 9140
For "Gang" implements, I'm hoping to drive my self propelled baler home tomorrow (more notes to follow). I have wondered if one could gang tow the self-propelled baler + another baler, but I would imagine driving the two would be tricky.