You have a Farm Pro FM5, a Chinese unit. I have a PDF of the manual if you want it. Parts are available from Homier Distributing Co --
FarmPro Parts - Homier Distributing Company, Inc.. I've found that contacting them by email (using the Contact Us link on web site) is the best way to interact with them. Phone service is dreadful.
King Kutter 5ft blades fit; just make sure you get the left-hand discharge type. Blade bolts are reverse threaded, as you discovered. (17mm IIRC.) I use an air impact wrench to get them off.
The blade spindles have oil seals on the top and bottom; too much grease will pop them -- the upper ones, in my experience.
The metal for the wheel strut channels is too thin. Mine broke in multiple places. I ended up drilling through the struts and deck, then bolted the struts to the deck, more or less permanently.
My unit came with true pneumatic tires, which routinely broke bead and deflated while mowing. I replaced them with King Kutter semi-solid tires and spindles, but I had to drill out the spindle holes in the forks to make them fit.
I've had to replace multiple idler pulleys due to bearing failure. They're sealed, with no grease fittings.
There's a Napa equivalent for the drive belt. B133 is the Napa part number.
This year, two blade spindles suffered bearing failures, causing the blades to tilt and scalp badly. As I was removing one of the spindles, its pulley shattered. Sigh. Farm Pro sells only the complete spindle unit, not the piece parts, so while I could cobble together the bearings (6205-RZ) and oil seals from different vendors, the pulley would be difficult to find a match for. I've decided to bite the bullet and buy two new spindle units (pulley comes with each unit).
All in all, it's not a very good mower, but it's hung on (with lots of work on my part) for 7 years. When something isn't busted, it cuts well. I've been up, down, inside, and out of this mower many times (never inside the gearbox though). I just bought a new 6ft KingKutter a couple of weeks ago, so I'm prepping the Farm Pro for sale.
Good luck. Holler if you have any questions about it.
Jay