I'm struggling to find good tools/strategies/professionals to help ballpark the rehab costs so I can make a reasonable offer. 

I want to buy a new private home and use my current primary residence equity (plus some cash) to buy a new primary house cash and then sell the old one.So, I am looking for a HELOC with little to no closing costs, as I will retain the money only for as long as I am selling my old house after buying a new one.Any advice?

What is a Cost Segregation Study & How Does it Work?

I understand the concern about using a HELOC for a down payment and the potential challenges with cash flow.

., the destruction threshold); which Property characteristics the destruction threshold percentage applies to, such as market value, assessed value, replacement cost, or unit count; for Properties with multiple buildings, if the destruction threshold percentage applies to each building, or all buildings as a whole; the replacement cost to rebuild per current requirements for zoning, and land use; the Property’s continued marketability, and economic viability; the amount and type of Borrower-maintained insurance coverage required per Part II, Chapter 5: Property and Liability Insurance, Section 501.02C: Ordinance or Law Insurance; insurance loss proceeds payout, compared to increased rebuilding costs, including from building code changes, Americans with Disabilities Act compliance, and the municipality's local zoning requirements (e.g., green compliance for new buildings, etc.); the sufficiency of estimated insurance proceeds from ordinance or law insurance and other coverages to repay the Mortgage Loan in the event of partial or full casualty, or condemnation; and for a Tier 3 or Tier 4 Mortgage Loan, if requiring execution of the Limited Payment Guaranty (Form 6020.LPG) would mitigate the risk of the as-rebuilt Property not supporting a Tier 2 Mortgage Loan.

Any agent who has worked with investors or owned their own rentals would have a good idea of the costs of maintenance and repair.

You had to looks at and confirm the lease terms (length and cost) as well as security deposits and so on. 

I also own a small renovation company that has operated in my are for nearly a decade. so my rehab cost on a property would be much more cost effective than hiring outside contractors.

You simply had to eat the costs.

We did the cost seg year one so all the tricky accounting is out of the way.