What’s required for success in this is deep knowledge of real estate in general and the area in particular; experience in real estate and the local laws, etc.; and capital to pay for options, marketing, legal counsel, accounting, tax advise, etc.  

The high end is the 20 or so co ops on 5th Avenue or Park Ave (one is on East End Avenue) that require liquid assets of $100 million + to gain board approval.  

It's definitely possible to get a loan as a foreigner but the rates will be high(er) and the downpayment required will be as well.

The insurance companies might require this. 

Duplexes are a little easier than 3-4 units with FHA since they don't have to meet self-sufficiency requirements.

Using a HELOC for the down payment on the second home might be an option, but lenders may require you to wait a few months post-closing on the multi-family before approving new loans.The BRRRR-style approach could work if you can renovate and refinance quickly, but make sure the timeline aligns with your lender’s requirements.

I’d love to hear any tips or strategies that have worked well for you in real estate marketing, especially with PPC campaigns.For those who’ve done this successfully, are there specific keywords, ad placements, or campaign types that you recommend?

I highly recommend listing to @Dave Meyer's most recent on the market podcast! 

Or I would recommend looking at out of state investing. 

Is there any form of education you could recommend?