First off I like your concept idea, I'm so used to seeing WoW style orcs/goblins and such. This character is more like a childeren's storybook character, very refreshing.
About the topology of the character, sorry I'm not a character artist so I can't say much but here's a link to a discussion about knees, elbows, and shoulders that might help.
I'm sure there are a lot of character topology threads here on Polycount. Just do a google search with site:boards.polycount.net
before your search terms. As Polycount's internal search function doesn't work so well.
As to your triangle count question, we need to have more information.
What engine will you be using for your game?
What is the target platform you intend to release your game on? DS? Wii? iPhone? PC? XBox360? PS3? Other?
If it's PC then what is the target specs for a PC for minimum and optimum performance? You mention a 5 year old iMac...
How many characters or dynamic objects will be on screen at once?
How large will the character appear on screen?
How much resources will be going into your environment/level?
Generally these are the kinds of things your programmer should help you iron out before you begin building your game models.
Another good way to figure out the answer to this question is to find a simlar game to the one you are making and disect it's tech/specs.
In the end you may have to just go ahead and test your game with test characters and environments made at different triangle counts/texture resolutions/shader complexities to find the right balance. This is probably what you should be doing.
Hope that helps.