The cost to evict, in terms of lost rent, vacancy, turnover costs, plus the work involved to secure a new tenant who ON AVERAGE will be WORSE than the one you would be evicting sounds counter-productive.

Nighttime security guard at a large complex in a Class B area.

I usually just send a screenshot of a bank accounts, but I have seen others get their bank to write an official dated letter saying they have at least X available to withdraw immediately.I would not consider the letter you attached as a proof of funds, nor a contract offer, its more like a letter of intent? 

I had a 5,000,000 unsecrued LOC and a 10,000,000 secured..

I live in Las Vegas which is a mecca for tourism, but new STRs here are very difficult to secure.

My hesitation is that I don't want a phase 1 that turns up nothing to lull us into a false sense of security.

The HELOC would likely be more efficient for securing the acquisition as it would be considered 'cash.' 

But I also want to give him security of having a lien on a property that is not the subject property.

If the land is part of the security - then you'd need to refinance the debt most likely.

Consider all the things that could go wrong and see if the lease addresses them: unauthorized pets or tenants, early termination, security deposit, lease violations, late rent, eviction, lawn maintenance, parking, etc.5.