Gas pipeline valuation

Hi guys,

I am currently interviewing for a role and have been tasked to build a valuation model for a gas downstream and pipeline player (i.e. they supply products and the associated infrastructure). 

I'm struggling on a couple of notes:

1) Typical forecast horizon - should I be forecasting out for 5 or 10 years?

2)  Would there be an exit multiple? Or is it safe to assume the end of life of the asset at some point

3) Would the typical O&G 10% discount rate be sufficient?

4) If the investor were interesting in acquiring a 10% stake for example, would that be just be 10% * enterprise value from the valuation?

Apologies, a modelling newbie here.