The sockets are metric:
8,10,12,21mm
Sparkplugs use a SAE 5/8" socket. These are specific for sparkplugs and are often longer than a "normal" socket. The sparkplug socket also sometimes has a rubber boot inside to hold a sparkplug when you are installing it. Many toolsets come with a sparkplug tool.
I agree with a 3/8" socket wrench. This is the size of the interface b/w the socket and the wrench. A 3/8" ratcheting wrench has a 3/8" square socket holder that fits into the back of the metric sockets listed above. Typically, you have 1/4", 3/8" and 1/2" socket wrenches. The larger, the longer the handle and the more torque you can put on a bolt. 3/8" should be enough to losen and tighten most bolts on a scooter. I find a 1/4" metric socket and wrench set very useful for many projects. Most tool sets will also have a bitholder allowing you to use your wrench with allen wrench bits too which I think the Buddy has a few of.
The 9, 13, and 14mm wrenches are also metric. These are what you picture when you think of a wrench. You can also get wrenches that have different sizes at either end (i.e. 12mm on one end and 14mm on the other).
4-in-1 screwdriver is just a screwdriver that has 4 or more interchangeable bits on it so you can swap say Phillips, Flathead, Allen bits quickly and not have to carry a bunch of different screwdrivers.
I'd also recommend a torque wrench (e.g. 3/8" drive from Harbor Freight) to make sure you don't over tighten anything.
I think any "Mechanics" set should get you started. In addition to the Pittsburgh tools at Harbor Freight you can also look at Craftsman and Cobalt from Sears and Lowes, respectively.
Something like this has most of what you need:
http://www.lowes.com/pd_338513-22328-85 ... facetInfo=
The bigger sockets you will have to buy as singles. Likewise the torque wrench.
