Hi Noel,
Here are a few things that I would consider in your situation:
1) 2-4 plexes in this price range are available, but not in the best condition. Typically you pay 25% more for a duplex than a single family home - you get more rents for a duplex, but you also have a larger investment. Depending on the home and the area, you can find good returns for single family homes. One of the advantages of having more units is that your vacancy rate is less likely to be at 100% at any given time i.e., if you have a tenant move out of a fully rented duplex, you still have some rent coming in; if you have a tenant move out of a fully rented single family, you have no rents coming in until you find a new tenant. Also, insurance is typically lower for single family vs multifamily. That being said, single family homes on the 150 range are not staying on the market very long. Having cash helps a lot, but you still have to be ready to make an offer as soon as the property comes on the market.
2) you mentioned that you are going to refinance in the "near future" what does that mean exactly? Have you talked to a lender about this? If for example you are not able to refinance for 1 year, interest rates might be at 5.5 or 6 percent instead of your current rate.
3) Your Cash on Cash return is going to vary based on your ability to obtain financing and favorable terms. Example: You invest 100,000 cash in a property with 10 percent CAP rate, so you are getting 10,000 per year. Lets say that you have the same property with with only 30,000 invested and pay 5% interest on the remaining 70,000 loan or $3500 per year. Now instead of making 10,000 per year, you are only making 6,500 per year, but your return on investment is 21%. Being able to get a higher cash on cash return is entirely up to your situation and how cheaply you can find money.
4) You have 10 percent for capital expenditures. What do you need to do to the property? Does it need a new roof or some other major improvement? Typically it is better to keep your capital expenditures low, as capital expenditures have to be depreciated over time instead of deducted.
5) in my opinion your vacancy rate is a little high, but that depends on the property. Before you invest in a property I would ask for the schedule Es for the last couple years. This will give you a much better idea of what to expect in rents. Every real estate property is unique - it is difficult and financially dangerous to make assumptions.
I hope this helps!