I recently bought a house to flip, and it turns out it will cost much more to renovate than I'd budgeted.

You may not know how to handle: tenant relations, maintenance requests, contractors, turnover, lease admin, property managers, "professional" tenants, neighbors, capital expenditures, municipal codes, federal laws, evictions, lawyers, budgets, taxes, etc. and how each one can affect the profits (or eliminate them.)

Are separate meters included in your renovation budget?  

I have already asked my agent to ask the management of the building about the cracks and plan/budget to fix them.

Make sure you budget for more "Expensive" repairs.

Seller owns a property free and clear and will entertain keeping the mortgage in place for 3 years.  

At this point the work has to be diy [ we've got some kids and grandkids our sons age with skills in painting, electrical work, flooring, sanding, etc] and the budget is tight.

Most of my thoughts are inline with what Karen Margrave has already told you, but here is goes anyway: Don't go a penny over budget.

I wanted to keep my budget to no more than $120000.

Yes, if it fits in our budget.