Hi Nitin, I'm following up with responses to your questions:

1. The decision regarding the sufficient legal income requirement will be made on a case-by-case basis. The authority reviewing your application will look at all of the evidence you provide, including your bank balance and income in the past 6 months from all avenues. Your income is not required to come only from consultancy work so can include interest from investments too, but the most important thing to remember is to prove with documents that you are truly a freelance consultant, in order to meet the conditions for issuing the DNV.

2. As a general rule you will need to provide written contracts, but e-mails and other documents can also be taken into account as a contract if you present them as such (as you can enter into a contract via an e-mail conversation). Essentially form of written evidence proving you provide services as a freelancer and get paid accordingly.