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?
It depends on whether you filed a provisional patent application or a non-provisional patent application. A provisional patent application will only be published if you file a non-provisional patent application that references it. Otherwise, the provisional will become abandoned after 12 months and never publish. If the information is within the public domain (published) an NDA may not do you any good. Of course, consulting with an attorney on that point is recommended.
If you filed a non-provisional patent application, it will publish unless you put in a non-publication request. In any case, an NDA is a good idea if you are entering into discussions regarding proprietary information that is not within the public domain. Having a non-published patent application on file helps bolster the NDA.
Using an attorney for legal issues is never a bad idea.
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.