There are an assortment of sockets required. They're all metric.
At the minimum, you'l need 15, 16, 17, 19, 20, 22, 25 and 30 (all mm). That's what I use mostly on my 790 which is, as the 990, built by Yanmar for Deere.
As far as the implements, it depends where they're manufactured. If made in Europe or Japan, the hardware will be metric. If built in the US or UK, most likely the English (American) sizes.
I suggest you get these sockets as 6 point, if possible, and in ½" drive. You can get a ½ to 3/8ths reducer for the smaller sizes.
Don't forget you'll need a torque wrench too.