I'm estimating purchasing both properties at $45,000 each, 1 all cash, 1 OWC. 

The only thing that can screw you is under estimating a rehab.

It was a primary but had to move out of state for work.Loan balance is $450K and estimated value of the house is $530K.

I'd reconnect with your lender to find out if their closing cost estimate was a conservative estimate.

I would estimate a 3000sqft home might go for $2600-3000/month.

It's hard to estimate for everything when there are many unknowns.

@Bruce LynnI agree with your estimate.

I'm pretty sure it's this one: Watch "Rehab Estimate, Scope of Work and Overseeing Construction" on YouTube with Andrew Syrios.

I'll give you the 3ish books that have been MOST instrumental to me in real estate investing:The Millionaire Real Estate Investor - Jay Papasan and Gary KellerThe Book on Flipping Houses and its cousin The Book on Estimating Rehab Costs by J ScottThe ABCs of Real Estate Investing by Ken McIlroyIn these pages you'll find few, if any, shiny objects of "the one thing that's going to make you rich forever", but rather you'll find proven, fundamental strategies that work and are prudent to build your investing career off of.

A full analysis should include (And you may be on your $100k estimate) holding costs such as Interest, Taxes, Insurance, Utilities, etc.