My question is, do you think it is important for the actual letters to be personalized to the PR & descendant address (mail merged with a CSV), or would it be ok to use non personalized generic letters (but 4 different ones), with the envelopes obviously addressed to the PR? 

I know I hang up if I reach voice mail that doesn't identify itself or sounds generic.

I'm reaching out to people who are dealing with a very specific, anxiety-inducing problem, and I'm addressing this exact issue in my direct mail piece (pointing out the problem, highlighting the consequences of their inaction, and offering them a solution that is simple and easy for them to accept).It helps to know exactly who you're targeting (whatever their issue is), because it allows you to put yourself in their shoes and tailor your mail piece to communicate directly to those issues (rather than writing a generic message that doesn't really speak to their actual problem).

Biggerpockets is a great format for generic info and you may find everything you need here. 

Use a 1" x 2 5/8" white generic label.

Also, instead of just using the generic message to the market of - "I'm interested in buying your house at 123 main st."

I have used the expertise of professional copywriters to develop letters that push the specific prospects’ buttons and persuade them to call you.The postcards are written to the prospect and are not generic.

My advice is dont do Yellow Letters or any other generic campaign unless you have the funds to go very big!

If generic I use a phone number designated for business card marketing.

But if you just go with the generic templates Michael Q just shared you could wait on the LLC and official business name.I'm wondering what others have done.