Now, if you have a holding LLC, and that holding LLC owns other LLC, you will only pay that LLET tax at one LLC level. 

There are state tax credits and offsets that help to alleviate double taxation, but the general gist is that at the state level you’ll pay taxes at the rate of the highest tax state whether that’s where the income is generated or where you are domiciled.

I would prioritize finding an accountant who specializes in real estate taxation over one that is local.There are 20+ accountants on this message board who specialize in real estate taxation.

But, this is based on the property tax at $6,000 and with a $2400 property tax I would probably purchase this property.

I would prioritize finding an accountant who specializes in real estate taxation over one that is local.There are 20+ accountants on this message board who specialize in real estate taxation.

The Suffolk County Comptroller does not take over taxation issues until a property is past due on its taxes.

I assume there will be capital gains tax at the very least, but this would still make my first property roughly 20% CoC which would be incredible.

You don’t get a debt basis, meaning your loss gets limited during tax filing.Also, there is taxation when you distribute property. 

I am trying to understand the taxation for overseas investors (from Taiwan and Singapore).

As long as the tax professional is familiar with real estate tax law and multi-state taxation, their location doesn't matter too much and it really only comes down to personal preference.