My husband is fond of saying...
"I don't like math but I like money!"
I am the odd duck that likes math and partials are all about math using the Time Value of Money (TVM) formula. You establish the cash flow, decide your desired return, and calculate the Net Present Value.
Once you are satisfied with the math then you have to document it. The purchase agreement will outline what happens if the note pays off early, what happens if the note doesn't pay, and who controls the right to enforce (and the asset serving as collateral). Not all purchase agreements are created equal and some favor the seller while others favor the buyer of the partial.
I also prefer to buy the most immediate payments but I have also sold partials where I retained the back-end or future payments. I suggest getting comfortable with TVM, using a good third party servicer, working with reputable buyers/sellers, and have a good attorney review the documentation.