So i put in an offer for 10% down conventional loan, $530,000 with 10k back to me for closing costs and it was accepted right away.

Whether to build on your lots now or wait depends on a few things: construction costs, rental demand, and your financing options.

Similarly, quality dividend-paying companies often retain some earnings to reinvest in their business, increasing the company's (and your) value over time.

Because we are financing most of it, when there are repairs, how would you factor those costs into our son's ownership?

There will be no assessment for increase in taxes or insurance so remains at $1935.

My question is, does anyone have a negotiation equation or strategy to get the price down, considering these costly repairs.

Keep in mind the potential for higher renovation costs due to older properties and the local landlord-tenant laws.Good luck! 

Its just hard to be good when they're chasing margin and keeping OH costs and personnel costs low.

Quote from @Michael Daniel: @Jaycee Greene these are low income areas however there's a lil room for rent increase but not much. 

Let's say at 6% over 30Yrs will be about $960 per month(Principle/Interest) with the remaining  interest only portion $40K costing roughly $200 per month. $1500- $960 + $200 IS $340Cash in your pocket after everything roughly $1500-$1160-$340 which is not bad for a 100% financed deal(INFINITE RETURNS)Now, If you ran this analysis on a 1% deal as @Nicholas L. mentioned you will undoubtably end up in the red each month.All the best!!!