The co-sign issue is definitely a personal choice. You just need to think whether payments will be made on time. Credit agencies report any late over 30 days and they stay on your credit for 7 years or more. There are also some credit lates that are worse than others, such as a mortgage late.
Another thing to help with your credit score is to keep lower balances on revolving debts (credit cards). They look at your credit line and how much of that balance is in circulation. If you have a $20,000 limit with less that $5,000 on it, that is better than a $20,000 limit with $15,000 on it.
I work for a mortgage company and see these types of things all the time.
Not an easy decision. I wish you luck.