Questions on doing a 3-5 P&L forecast for a SAAS company.

My project is doing a 3-5 year P&L forecast by creating revenue and cost driver assumptions. I don't have any historical data to base my forecast off of, so I need to build-out from scratch using some customer data (i.e. additions and churn) and trying to justify my subsequent assumptions. I had a few questions about building out my pro formas:

• When you do a 3-5 year build-out, do you have to discount net income back to present value for future years? I guess the better question is: in what cases do you have to discount for TVoM and when do you not need to?
• If I don't have historical data to work with, is it fair to base my G&A cost assumptions as a % of revenue? If not, what's a better metric to base this off of?

Sorry if my questions are a bit elementary but I would really appreciate any insights into building out a more comprehensive and coherent model. Thanks in advance!