I've been practicing with analyzing properties but don't have any experience in estimating repair costs. 

Do I ash for comps for EVERY property that I want to analyze or do I ask for comps after I do my first round of analysis and I want to do a more accurate analysis. of a potentially promising property?  

I analyzed the deal and want to offer $70,000.

I analyze properties for investors nationwide, 20% discount in that area is good

I've been learning, analyzing and growing my knowledge like mad the last 3 months.

We aren't afraid of the risk, we don't want to be foolish however, so we very much welcome anyone that would like to analyze our thought process and give us feedback.  

If the analyzed numbers make sense at the listing price (ie, $250 cash flow and 14%CoCROI), would you still make a lower offer?

Phillip had a great point, I'd also want to break down the individual expenses to get a more confident estimate (if you haven't seen it, Brandon did a great YouTube video on estimating expenses & cash flow with BP's Rental Property Calculator, called "How I Quickly Analyze a Rental Property for Cash Flow").

Educate, Analyze, Educate some more, then pull the trigger.

You may find yourself understanding the following in greater detail - Wholesaling, Understanding Assignments, How to analyze a flip, How to analyze a Buy & Hold, Understanding Lease Options, Understanding creative financing, How to put a contract together, Understanding how to negotiate a deal, and more.