Price will be dictated by BPO...

Don't let any rule of thumb dictate what you should and shouldn't buy. 

Question though: does this agreement say she gets $300k of the sale no matter the price or does it dictate a 60/0 split?

State law will dictate priority of said lien.  

This will somewhat dictate what kinds of properties you can consider...You can certainly link up with some realtors and they can show you stuff that's listed on the MLS...But some of the best deals are off-market, and for those you'll either have to either do your own marketing, or buy through a wholesaler.

Can title companies dictate this way?

Neither you, nor a buyer's agent, gets to dictate the commission paid by the seller to the listing agent, this is set by their listing agreement. 

I currently own a vacation rental property and we were able to dictate our terms during the purchase.

I like that the value is derived from the NOI and cap rates in the area - that gives the owner a lot of ability to dictate the price by lowering expenses and maximizing revenue.  

I have never heard of any hard or soft rule which dictates a 1:1 ratio of loan to net worth.