"You shall be responsible for all costs incurred enforcing the terms of this Agreement."
You are only willing to be responsible for the costs you have to protect yourself. You don't ever sign an agreement that says you'll cover all costs for litigation. That's probably just a mistake in the NDA.
Also, you're sub-contracting in this situation. Be sure that's what you want to do. The situation you're describing is one where PartyA, the contractor, is getting you to relinquish your "employee" rights by making you a sub-contractor. But they keep all benefits of being the actual contractor with PartyB, the client. This is normal sub-contractor stuff. But your small business needs to put responsiblity for most problems on the prime contractor. Don't offer to protect them. They're benefiting the most in this situation so they should be shouldering most of the burden.
Keep in mind if you offer to be a sub-contractor, you cannot go behind the prime contractor's back and solicit the client for direct work. That is stabbing the prime contractor in the back and any retaliation by the prime contractor is fair play.
You should set up an LLC or other legal entity to contract through. And no, just having an LLC doesn't shield you from legal action. If you have to go to court and you would if someone sues you. The courts will want to know who, if not you, is responsible for any mistakes you made. Having a separate legal entity is to prevent someone that does none of the actual work from being responsible for mistakes made by sub-contractors, employees, etc. Keep in mind, if I'm not mistaken (I'm not a lawyer), a state has the right to refuse to grant you a dissolution of the LLC due to pending legal action(s).