Would you say 5% would be sufficient?

However, I would caution you on quality over quantity. 

You are looking for a combination of 3 things: sufficient vacant land, residential and commercial, all in 1 property and affordable.

Since you have no significant debts and manage your finances well, your existing financial data from tools like Fidelity’s Full View should be sufficient.

If there’s sufficient evidence to conclude that a deal can be made, it may be necessary to go through another round, or even two of offer - counter offer before reaching an agreeable price. 

If I was evaluating that property, a couple of things would stand out: I agree the 5" slab probably isn't sufficient given the potential for freezing and movement under the slab.

You do need to send in a check with the extension, the irs provides guidelines for calculating the amount.Real estate investing complicates taxes sufficiently that I have found I save way more than I spend by having a CPA who specializes in real estate investor tax returns.

Quality over quantity.  

Your primary insurance policy with $300,000 in liability coverage should be sufficient in 99.999% of all lawsuits.5.

They don't care about your job really, to them it is about quantity, not quality.