Once you have the cap rate for the area you can determine the value by multiplying the annual NOI by the cap rate.  

Because electricity and gas are monthly expenses they need to be multiplied by 12 to come to the annual amount.So $200 electric should be $2400 and $500 gas should be $6000 total for year 1.I would also highly recommend that you transfer the cost of both electric and gas onto the tenants as this would decrease your annual expenses.

Am I simply going to multiply that by my purchase price in order to calculate my property taxes?

You multiply the monthly income by 50% to estimate expenses.  

That way, both the loan paydown and appreciation are 'multiplied' by 2.5.

The way you calculate your weighted average interested rate is by multiplying the rate of on each debt by the proportion of that debt to your total debt, and then adding up that sum.For example, say your 15k debt is spread at 4k Car, 5k CC, 6k Student loan.The car is 4/15, the CC is 5/15 and the SL is 6/15.  

Now, while Gross Rent Multiplier (GRM) is taken into account for rental properties.

@David VanSteenkiste my only suggestion would be to get some bids and perhaps apply multipliers to what you find.

Multiply your square footage by comp square foot and that would give you a decent idea. 

Then calculate your maximum offer price by multiplying the ARV times your required discount and then subtracting rehab.