If you are going to buy a property as a BRRRR you will generally be able to have a much more detailed, much more accurate estimate of expenses, especially in the short term (5-10 years). 

What's a good way to test the accuracy of tools like the BP rent estimator and others?

Estimated that from SmartAssest property tax estimator. 

It was in poor condition with some of the following problems:No hvacPast termite damageWater damage in multiple areas with active leaksAluminum wiringPoor layoutGrading issuesFoundation issuesWe did some numbers and estimated the rehab to be close to $100,000.

But from my initial DD, their estimates of repair costs and comps used seem to have wild ranges.

The house was purchased for 203k, our remaining mortgage is 191k, and the current market estimate (although not an official appraisal) is 276k.Our goal is to accumulate properties through the BRRRR method, and as I see it, we have two avenues through which to do this:I can simply use the money in my account to BRRRR.I can pay off my rental property in Beaumont, TX, and utilize a HELOC to BRRRR.I know that most people will say that they never want to pay off a house due to the tenant paying it off for you, and the difference in percentage returns that you get based off of appreciation, tenant payoff, etc.

Different websites estimate between 5% and 10% appreciation in a year!

According to Zillow, they estimate I could rent my home for $2800 per month.

Do you have a list of what repairs you'd be doing, and estimates from licensed contractors or materials suppliers on the costs?

Get good at estimating rehab.