My suggestion to anyone asking the question of "what endmill should I buy?" is pretty simple:
Buy an 1/8" 2 flute endmill.
This might seem like it's being over simplified, but here is my rational:
When I started into CNC several years ago I was paralyzed by the endmill decision. There are so many out there and there isn't much information suitable for a beginner. It's very intimidating: number of flutes, coating, speeds/feeds, etc). So I spent months and months (literally) researching what I needed to buy and trying to learn as much as I could about these mysterious tools. In the end, I just bit the bullet and ordered one, mostly out of frustration. "Just jump!" is what I thought.
Limiting yourself to one endmill is a constraint, and although the connotation of the word is somewhat negative, I like to think of constraints as something helpful! Constraints keep you grounded and don't allow you to get sidetracked with unnecessary action items. Constraints are good in my book. They let you focus on what's important by not giving you so many choices. For instance, when you are designing something to cut, instead of focusing on what bit you will use, you can instead focus on the design of the part, with your trusty 1/8" end mill as the constraint, you will have to focus your design to accommodate.
With an 1/8" 2 flute (center cutting) endmill, you can machine all the plastic and wood you want, and will do a nice job staying cool and clearing chips. Get a HSS (High speed steel) and save yourself some money. You might want to buy 2 or 3, just so you have extras.
Once you're comfortable using the machine, and you can see for yourself where another size/type of endmill would be more beneficial, that would be a good time to branch out and try a few other types.
I started the wiki page a while back: http://www.shapeoko.com/wiki/index.php/End_mills
. It's by no means an all encompassing list, but it's a good place to start.
Shapeoko #0 - a couple of upgrades.