I never lock any HELOC money into any project that I do not plan on getting my money back within 6 months.My HELOC is right around 9% as well and interest only.

Educate yourself on the basics of analyzing property.

So there are a few ways to look at this, the 1st question is are you sure your renovation’s will double the property value, most renovations on primary residences, your lucky to get 60-70% of the dollar cost back out upon sale, so really nail down your comps, than you have basically a math problem if your rate on your primary goes up how long if ever before that extra payment is more than 400k in taxes, that’s just a simple calculation to help you decide, but I suspect you actually have a different question here, from a purely financial perspective my guess is the best option is to do anything to avoid that tax hit, but one of the reasons to make money is to spend it on things that you enjoy, based on your overall financial picture and a subjective view of how much enjoyment you will get out of a renovated home, you should decide if you want to roll your profit into another deal or “cash-out” your winnings, I love cars and I’m willing to spend more than is fiscally smart on them because they bring me joy, there is nothing wrong with that, but i don’t think it’s a fiscally smart choice, without knowing your exact property id imagine a renovation would fall into that category, so that’s the terms I would use to decide.

Since you do have the land outright, there’s certainly a way to make something happen here.Best of luck with your project.

Quote from @Conte Cuttino: My advice is very basic: always hold a property unless you can sell and invest the money in something that provides a greater return or moves you closer to your goals.

Basically I have no clue what I'm doing yet but I'm excited to see and learn.  

I’ve only done one basic exchange and Dave did it for me. 

What are your projections on how long the rehab will take, and what do your reserves look like in case the project runs longer or goes 10-20% over budget?

You'll bring a downpayment based on the total project cost and you can get seller financing to help with some of the equity injection. 

I have a "notebook" (that's basically a file folder, in Evernote) for different types of info I want to have access to later: lender info, referrals for service providers, property listings that I come across and want to check out later, websites that have demographic info... all sorts of things.