I haven’t purchased my first property yet, and having an accurate idea of what a property needs for a rehab, and how much it would appraise for is important to me as I plan on using the BRRRR strategy.

Do yourself a favor, join a LOCAL rental housing owners association.They have the forms you want at a cheap price and usually have attys to review/renew them.As a landlord, accurate paperwork is your best defense.

You would need to check with your state's licensing department and contact them directly to get the most accurate answer. 

Here is my math, if you see any suggestions for my calculation to be more accurate let me know.

There is no HOA because there is only one owner.Corey, clearing up these misconceptions will definitely help you more accurately calculate the cashflow on a property.$300K is a lot of money, no question.

Does anyone have a resource or way that they get a good idea of how much house they can afford with their income and credit that is somewhat accurate without having to do a full inquiry?

Get with your CPA who can help you get the accurate answer. 

Given that you're already to the point of having an offer accepted you're going to want to develop a very clear picture of the actual expenses measured against the income to be more accurate in terms of what you could actually expect for cash flow.I think it comes down to what the house is currently worth versus comps regardless of what it was listed for since the offer was accepted only $4,000 below their asking price at that time. 

This could make it easier for your buyer to know what responsibilities they'd be taking on with it.Lastly, I'd talk with an experienced broker/agent who could give you an accurate BPO or CMA on the townhome. 

Note, not all GIS measurements are accurate.