I submitted software patent to USPTO. In today's climate probability of getting such patents is very low, so I decided to treat it as a trade secret for now. Do I need NDA when explaining the gist of my invention to potential investors/customers, and if so should it be written by lawyer or version downloaded from Internet will be sufficient?
A trade secret and a patent are mutually exclusive choices. An NDA would almost certainly be needed for the former (since it's secret), but not really for the latter (since it is, or will shortly be, public). It really comes down to which route you decide to pursue. Of course, there is no real disadvantage for you to use an NDA (except to the extent it dissuades potential investors or customers).
That said, if you decide to use an NDA (and for your purposes, there is no disadvantage to it), it really should be prepared by a lawyer. You have no idea of the quality of a version from somewhere on the Internet, or event whether it is even compatible with the laws of your jurisdiction.