I won't have time to get an assessment done so I'm trying to do my best and get a conservative ball park number for the remediation. 

I suggest you self assess and see what areas you're weak in and learn that 

This assists the seller in not feeding the house.A licensed property manager gives a written rental assessment of the market rent of the property, and this is placed in the seller's file.A credit improvement company with underwriting experience and knows the Fair Credit Reporting Act (see ftc.gov) assists the tber in increasing their FICO score so they have a better chance to getting the mortgage than if they were left to their own devices.I hope these ideas help.

Then look at the assessed value that the tax collector is using.

I think your agent is not a property tax expert.You want to look up the process yourself for your area and see how it works.You also want to investigate HELOC's and any collection repurcussions in your state versus a regular loan if you default later on.Some properties are under assessed and not over assessed so just make sure you have a strong case or your tax bill could go up instead.

It also ensures one has the ability to pay dues and assessments.

Do I try to put down an offer with some kind of option period to lock it in while I do due diligence on the property (bring in a contractor to assess rehab while I continue to evaluate rents, loan options, etc.) or do I try to arrange an in person review of the house and assess rehab prior to making any kind of formal offer?

I'm trying to build an excel spreadsheet to help me quickly perform assessments and this review is tremendously helpful!

I've read over the 2% and 50% rule and they make sense, and obviously are techniques to "screen" or do top-level analysis to help quickly triage a deal.As I've crunched numbers on hypothetical deals I've always pulled out PITI (principal, interest, taxes and insurance) out of the monthly rent to assess what the cash flow might be.

No harm in trying to see how to get taxes lowered (you'll still be assessed for the value of the land at minimum, but some reduction is better than none).