At the time, his offer was low, and I had potential plans for that space, so I turned him down.

I plan to use a HELOC but would be interested in the thoughts of others in BPs on this approach or recommendations if there are other ways to financing that I should consider.

These are all things I am trying to plan for. 

This is who will approve or deny your plans when you apply for a permit.

If you plan to be there for the entire rehab process, then go for it. 

I feel like the seller would wait and I could just buy it in cash since I do plan to refinance it anyway and pull out the equity once I’ve done some repairs/upgrades.

What kind of investments are you planning on purchasing, and what area? 

That will involve settling on your plan for renovating.

If you like help structuring this deal differently to give you more leverage, let me know, I've worked with new investors on game plans and seen others start with 90% LTV on their very first deal.

Private Money Lenders are going to be hard for a primary residence as it is an investment and typically they want to see a return prior to two years - also, the house would need to be purchased at a discount which would limit your options on a primary residence - now if you are looking for something that needs some work that you are planning on fixing up, it would be a little more feasible.