Do not take the money out of your 403(b) account to purchase this co-op. IF your not 59 1/2 , you'll face a 10% early withdrawal penalty, plus the money you withdraw is taxed as ordinary income. If you do a loan, it's like being taxed twice on the money.
Think about it:
1. Your being taxed on the money you withdraw
2. You pay back the loan with after tax dollars
3. When you retire, your taxed again on the money when you take it out
a 403 account is a retirement account. Do not touch it!