They have a HELOC of ~$60kNumbers:Comps suggest ~$199.5/sqft (max 2,020 sqft)Estimated ARV: $335k (based on comps)Potential rent: $1,975-$2,000/monthQuestion: Do these numbers seem reasonable for the Braselton market?

For example, when I get up to “estimating rehab costs”, for me it’s like it’s impossible to know how to estimate rehabbing costs without reading an entire book on it.

Another out-of-town real estate enthusiast, using local comparables, estimated an After Repair Value (ARV) of $40,000 with high confidence.

They can point out everything they notice and give you estimates for operating the property.I also like to include a couple of checks in my underwriting to make sure I'm being reasonable.

Is there a recommendation on how to estimate these numbers?

I would not be surprised if an expansion like this was estimated at 5 - 7 months and for it to actually take 8 - 10 months. 

To answer some of your questionsGross monthly cash flow is $3200 rent - $2550 expenses = $650 Yearly cash flow= $7800Vacancy 8-10% is close to 1 month's rent so = $3200 yearlyRepairs: 5x rule = $576 yearly CapEx: 1% of property value yearly= $4,000 (includes Special Assessments)Once I deduct vacancy, yearly repair budget and capex estimate I'm breaking even, but is that real?

To estimate offer price after showing, do you know what are the things to pay attention to look?

Cashflow is what's left over after you factor in repairs, capital expenditures, vacancy, property management, principal, interest, taxes, insurance.So it sounds like your not running numbers correctly - underestimating CAPEX which leads to over estimating cash flow. 

General terms:80-90% LTV/ 100% LTC with a cap at 70% ARV9mo note at 12.99% I/O3pts Origination$1k UW$1k LegalAdditional costs - title, appraisal, property insurance (~$2k)10% of renovation estimate as contingency due at closing.Renovation funds issued on a reimbursement basis for work completed.Looking for constructive feedback.Thank you to all that contribute.