It is best to form the LLC first so you can purchase the property in the name of the LLC. However, this limits your financing to commercial loans, because lenders won't give you a typical conventional residential loan under an LLCs name.
Some people prefer buying SFRs and 2-4 units using residential financing, so they get a loan and buy in their own personal name then transferring to their LLC later. However, it's technically a breach of your loan agreement because residential loan agreements have a clause in them that says if you sell or transfer title, the lender can call the entire loan amount due to be paid in full immediately. Google "due on sale clause" to look up the risks. That's why it's preferable to form the LLC and take title in the name of the LLC first. You don't run the due on sale clause risk. But your financing is limited to commercial financing.
Also, if/when you go the LLC route, you MUST keep rental profits and losses separate from your personal accounts. I'm glad you prefer to do it that way anyway, but keep in mind it's not about preference, its important so your LLC remains legitimate in the eyes of the courts and you will retain your liability protection.