Rental apartment modelling

I literally have no work-experience from RE but it's been a dream of mine since I was a kid. After stacking some annual bonuses I feel its time to get into the RE market. I'm completely against investing in something I don't understand and haven't made calculations on, so I'm trying to model whether or not a rental apartment is worth the investment and has come across a couple of question marks I hope you guys could help me with:

1. Is there any point of discounting the NOI? I mean why would I? I'm not valuation the actual apartment since it already has a price I've bought it for and all future cash flows are probably being used to amortize on the loan.

2. My investment horizon is 3-5 years - what is a reasonable MM (money multiple) - I'm going for atleast 2.0x (same as for PE investments)

3. Do you guys make any assumptions that is good to know of?

4. Do you include tax deductibles in your estimations? I mean you could? And it would give better and more correct figures

I appreciate the answers!!