If you're looking at a flip scenario, the assessed value of the property has no bearing on your numbers except that it determines how much property taxes will be paid. I successfully petitioned the county to lower the assessed values (and therefore the property taxes) on my personal residence, rental property, and flip property.
Instead of assessed value, you need to look at good comps for the area to see what homes are really selling for right now. The home could have sold for $1.2M last year, but if they are only selling for $120K right now then that's where you're going to get your ARV from.
Where are you getting rental comps from? I'd be sure to call the "for rent" signs in the same neighborhood or as close as possible to get an idea of what kind of rent to expect. Rents for a general area or city can vary greatly from what the home will actually rent for in that neighborhood... for better or for worse.
Also, you'd probably be well served to get a contractor or two to walk the potential properties with you to give you a realistic idea for rehab pricing. Don't expect them to do it for free, especially if you don't have the property under contract.
Good luck and keep us posted.