I decided to hire a full-time maintenance staff which is definitely helped reduce costs and avoid the issue With unreliable , bad work.

If you can time the market 55% of the time, I'll hire you as my financial advisor, we can both get rich trading stock options!

I'm at 64 units now but I hired third party management.

How should I approach the seller(direct mail or hire agent)?

If it’s not urgent then it can wait a day or two for one of your schedules to align which is what we have to deal with when we self manage vs hiring it out so that either you or the tenant is home

If landlord had to pay $1000 to advertise the unit or hire an agent to get it rented, the original tenant is off the hook for these costs.

I don't know what your background is @Zee Ayezain but if they don't want to sell it to you with owner financing as @John Underwood suggested I would recommend to them they hire a property manager. 

A great place to start is with the county tax assessor or to hire a valuation professional to appraise the property at the time of purchase.

I Hired the wrong contractor to finish my basement.

If you just hire the cheapest or first PMC you speak with and it turns into a bad experience, please don’t assume ALL PMC’s are bad and start trashing PMC’s in general.