I'm in the process of making a 125 into a "perfect" budget cafe (good disc brake, cafe seat, clubman bars, 12 volt conversion, good tires, clean overall look)
any moped clubmans work. You don't need the horns to clear the forks, but they should.
As far as seats are concerned, just get your stock one modified. I have an original seat, a "flat" pleated track seat, and am in the process of building a surfboard-style laminated wood solo seat.
The 125 is a great first bike. It's small, simple, easy to work on, and you can get all the parts from the dealer for pennies.
I'm doing all my mods on a budget on the back porch, check out my thread if you have questions, I sort of touch on everything.