Do I charge extra rent in case she not telling the truth?

Whatever thickness you get, try to get it with padding so you dont have that extra cost in material and labor

For example if I do a fha loan and my down payment is around 5k, how much extra should I have saved for this ?

It affects all their income not just the marginal return a few extra units can bring.Anyway thank you all, this has reassured me of my strategy.

Refi will cost you $$$ and you may need the cash to continue improving the property or paying more taxes if the reappraisal increases your tax base Also you may need the extra cash if a tenant can't pay rent. 

On my end - I've heard that some of the easiest (and potentially cheapest) tricks could be:- applying a fresh coat of paint on the exterior / interior living room spaces of the home- refurnishing doors / door knobs - updating the lighting of bathrooms or kitchen area light fixtures- bathroom refurbishment- curb appeal / clearing out excess weeds/trees/shrubbery.I've also heard of situations where the home owner would either offer to build, or provide a backyard shed for the tenant as additional storage space, charging an extra $100-$150 per month if the tenant wanted to use it. 

I want to open a side business due to the extra income I am already making.

On top of this I got charged an extra 10% on the invoice price  Should I be worried about this and start interviewing new property managers or am I over thinking things? 

Hi Pedro,I would recommend looking into a property that has a detached garage or extra space in the lot to add/convert an ADU, that's the safest way to cashflow.

So if you can cut that down by $1000/month, it's like making an extra $1000/month.