Help me adjust my expectations - first deal pending

I'm a complete noob. I'm planning to put in my first offer on my first rental on a SFH in Kansas City in the next day or two.

$140k offer

8% interest rate

20% down

8% management fee

5% vacancy

5% maintenance 

5% cap ex

I haven't gotten insurance costs yet but it's been suggested that I plan for $2k per year. 

Once I plug in all those numbers, I'm pretty close to breaking even. I have the ability to put 25% down or more to increase cash flow. I just want some outside eyes to help me understand if this deal is what I should expect to find as I build my portfolio.

I plan to buy and hold long term. I am expecting to do 3-4 deals per year until I get to 20 properties or so and then snowball the debt so they are all paid off by retirement. 

Am I ok with this plan to ride up the appreciation, mortgage pay down and rising rents? Or is this type of deal that you savvy investors out there would scrap and try to be more aggressive? Any thoughts are appreciated!