gat_sby
I find the idea of using your engine as a aggregator for github very interesting though. I am not familiar with the github star and watcher apis but there might be a way to push upvotes/favorites/likes to github through interaction on hacksy, though you would still face the battle of bring users over.
Agreed in that a good designer and front end developer will run you between 80-100/hr but you also have the option of scouting a younger designer who is willing to work for a little less. Now that suggestion is against everything I believe in terms of payment standards for freelance designers, however there are so many ridiculously great young designers that are hungry to grow their portfolio and get some great experience in real world application - hell, thats how I got my start years ago.
My last suggestion is to consider contracting for a fixed project payment rather than hourly. You said that you're looking for a part time designer, but looking further down the road it is a lot easier to plan your budget accordingly, and the stability for the designer ensures quality work.
And of course, good luck!