I have 1 hint for you, and 1 question from my sister:
Hint: I would begin writing the CORE of the game to your needs.
(client-server chatting, objects, etc)
Skills can build up on objects eventually, (or the other way around, depending on your game)
So you should write up a list of what needs what.
If your skillsytsem is dependant of an item system, you need the item system first
As you are writing this game more hobby-like,
I'm not sure if you have planned any structure for coding or so.
Now the question:
I have a sister who has begun pixel art a short while ago.
She is not "good", but she is useful to our team to an extent.
She pixels many Inventory Graphics for Items we have, and also attempts other stuff.
Though as our Game requires high standards
(not the highest, but "relatively" high) compared to some other projects here GFX-wise.
So, how skilled must your pixel artist be?
What quality do you expect? what experience?
atm her PC is broken anyway (the monitor is fried, think its replaced soon)
I suppose she would work for free on a specific level of quality,
to gain experience and get better.
But this is not clarified with her herself yet.
(after all, as shes not too good yet, i want to ask what quality level you require first)