2 - Right now, with you that profit buried in this property, if you sold, could that same profit now used as a down payment, buy you a larger property value? 

I'm a residential loan officer and I have a client who is taking out a loan (not through me) to buy a wharehouse for their business.

I ended up buying a duplex in North Andover, only 30 min drive from the city.

These are EXACTLY the kinds of property management challenges you are trying to take on by buying a rental property!

Many reason I stay away from Condo's including:HOA regime feesUnexpected and expensive HOA assessment fees for upgrades or repairs that you may not agree with.RulesClose proximity to others that could cause disagreementsSaturation and competition on renting condo's ParkingI would personally never buy a condo for these reasons and look for a deal on a house.

What should I be careful about compared to buying an existing property, and are there any key differences? 

I am looking to buy a fixer upper.

The year's lowest priced small multifamily sale on Long Island is $430,000.If you want to househack a multifamily, you'll want to find ways to boost your buying power.

Hopefully the reasons are obvious.I've already outlined the type of deal I'd buy if buying in the Midwest today.

Just like anything it comes down to doing your due diligence and inspecting the property before you buy it so you are aware of the age of the capex and all the mechanicals.