What to do if I just moved into a condo and found out that it needs significant plumbing work?

You could file one lawsuit in which you would sue the seller for fraud and the real estate agent for negligence.

Fraud is the intentional misrepresentation of a material fact made with knowledge of its falsity and with the intent to induce your reliance upon which you justifiably relied to your detriment.  In other words, you would not have purchased the condo had you known of the plumbing problems.

Fraud also applies in cases of nondisclosure where the seller did not disclose a material fact which the buyer could not have reasonably discovered.  You could not have reasonably discovered the plumbing problems prior to purchase.

Your damages (the amount of compensation you are seeking in your lawsuit for fraud against the seller) would be either the benefit of the bargain or your out of pocket loss.

Benefit of the bargain means that a defrauded purchaser can recover the difference between the real and represented value of the property purchased regardless of the fact that your actual loss may have been less.

Out of pocket damages for fraudulent misrepresentation permits recovery of the difference between the price paid and the actual value of the property acquired.

Your lawsuit has separate causes of action (claims).  One cause of action is for fraud against the seller.  The other cause of action is for negligence against the real estate agent.

Negligence is the failure to exercise due care (that degree of care in this case that a reasonable real estate agent would have exercised under the same or similar circumstances to prevent foreseeable harm).  Your damages (the amount of compensation you are seeking in your lawsuit) for negligence would be the cost of repairs to the condo.  You will need to mitigate (minimize) damages by selecting a company to make the plumbing repairs to the condo whose charges are comparable to other companies in the area doing plumbing repairs.  If you fail to mitigate damages by selecting the most expensive plumbing repair company you can find, your damages will be reduced accordingly.