This is the same exact result as if you did not sell it.This is assuming that you're buying depreciable property as a replacement and not, for example, undeveloped land.

When proposing a deal to potential investors, how many investors is ideal to say fund a $500,000 duplex intended for buy and hold? 

With its proximity to NYC, numerous public transportation options, and overall vibe and attraction, it is no wonder why the popularity has been reflected in the buy-and-hold rental market.

Also do you know what you are buying?

You could deploy that money elsewhere and buy a few  smaller (or at least less expensive) homes in a friendlier market and come out way ahead unless you are content with just waiting for the market to appreciate.  

@Adam ShaIf you can buy a home for cheaper than rent then that is a red flag.

Buy and hold notes (performing and non performing)3.

If you were able to buy 2 duplexes as replacements then you would own MORE property so that any market appreciation is magnified and you might have MORE loan principle to pay down further building wealth that you don't see just by looking at cash-flow. 

MY SUGGESTION: invest in yourself by buying hard copy books and training on the foundation topics you wish to develop skills (RE legal, title, escrow, financing, notes, etc.).

Personally we have long stopped buying "doors" - we want as much revenue with as few tenants as possible, so we buy higher-priced homes in the Milwaukee suburbs.Last suggestion: look into DST's.