Understand, real estate may not be the solution you are looking for.

This used to be a near-impossible task for a company like BiggerPockets, but technology and 3rd party solutions are getting better.

One solution I’ve seen is closing the purchase in cash, then securing the HELOC afterward, though this carries more risk.

The only solution I see is connecting them to city sewer which won't work 95% of the time. 

Can anyone recommend good companies to root cause  and implement solution to fix this issue?

But if your budget is tight or your home only needs minor updates, your real estate agent can help you think outside of the box by suggesting simple DIY staging tips to help your home look its best.Bottom LineStaging your house properly can make it much more attractive to buyers, but it’s not a one-size-fits-all solution, and every home shines differently.

Is replacing the HVAC with a completely different unit the best solution?

Also be open to knowing that you are acquiring a series of problems.You're in the solution business. 

We are in a similar boat, however I have not yet found a solution for less than $5K even for above the ground.

It’s a cleaner, more reliable solution, and while the higher subscription cost can feel like a drawback, it’ll save you a lot of headaches and time in the long run.