It depends upon you whether to sign NDA or otherwise. If you are willing to sign NDA must ensure to understand its terms and agreement, obligations and time period. In any case, your dues and relieving letter can not be held for want of entering into NDA. Your relieving is subject to the terms and condition of your appointment letter. Please confirm, whether the provisions of NDA was mentioned in your appointment letter. If not, then you are not under obligation to sign NDA.
Starting a new software company with the client of your ex-company is no bar unless you are restricted to do so in writing earlier.