Upon the tenant leaving, they discovered the slow leak he had "Fixed" was rerouted between the wall and the cabinet and it rotted all the subfloor in the kitchen, about $6,000 to fix. 

The tenants talk and if one has a new A/C, water heater, fresh paint, carpet, new cabinets, etc. then there is a higher rent value for that.If the tenant leaves with the more run down units how much money per unit would be needed to get to the condition of the newer tenants units commanding more rent??

Cheap cabinets.  

A book could be written as to the structures as you have many options, they can buy and finance and you contract, you buy and sell turn key, you can get the seller involved, do the work for them and flip the contract to an end buyer, partner with the owner and sell turn key, arrangements are endless.To structure it, you'll need to consider the buyer's position and ability to buy, they will be getting the end loan.Really boils down to financing, who gets the construction financing, does title need to pass  to do the work, where do you put your profit, again, the end buyer's ability to finance.You'd be selling from plans and specs, yes, you need a good down payment if the buyer is picking appliance, colors, materials as what they may select may not be as marketable if the fall out of the deal, you're stuck with pink bedrooms, green carpet, crappy ceiling fans and hickory cabinets.

I'm looking around the place trying to assess what it would need and remembered meeting a guy in the same complex who did a complete reno and said he put about $15k into it, (complete kitchen gut job, tile, carpet, bath cabinets, paint, moldings...the works) So I factor this into the equation.

The houses I'm looking at look like cosmetic fixers....paint, new cabinets, landscaping, etc.

It is on 3.2 acres, with a wood burning fireplace, 4/2.5 baths, wooden fixtures, large deck, real wood cabinets, storm windows, 2100 sq ft, there is a large workshop on the land, with an office and bathroom located in it.

I'm not talking about spending money where it isn't needed (replacing perfectly solid cabinets for example just for something more stylish).I'm sure there is no right or wrong answer to this, but I'm wondering what you opinion is on updating a house and considering it as marketing.

Never have had to replace doors, moulding, cabinets, appliances, etc. due to pet damage. 

The new kitchen layout offered much more of an “open concept” by removing the upper cabinets above the bar.