If a roof needs to be replaced in the 2nd moth of owning the property, and it costs $10k to replace, you can't simply put your own $3k in to get it fixed.

It is the selling price (less. closing costs and fees) minus your purchase price along with capital improvements you made (new roof, bathroom, etc).You cap gain/profit will be less than the $28k difference.

I am looking to gain knowledge in all fields of real estate but feel wholesaling would be a good thing to do while I gain knowledge in other aspects of real estate.

Often times 1 mo rent for new leases, 1/2 month rent for renewals.What is the condition of the HVAC, water heater, roof and gutters, patios/decks, fencing, etc?  

You should take each major item (roof, HVAC, exterior paint, etc) estimate replacement cost and lifetime and use that as Capex reserves.

Tesla Roof???

On a safety aspect I would personally want to install central AC & Heat.

The inspector has just confirmed it will need a new roof.

He found it on Zillow and it needed a new roof so my husband negotiated the price from $190,000 to $180,000 and received 5k at closing.

Obviously building new eliminates a lot of the cap ex concerns that you would have with a 100 year old property (roof, foundation, siding, etc).