You can get the $20k from the bank day in and day out.

As an investor, and with others teaching and mentoring, a person in real estate should never leech off of a host.

The bank is going to want to know that.

Banks will be reluctant to lend you and other buyers/borrowers a mortgage for a building like this as it is seen as high risk.

My understanding is the key is to keep separate bank accounts and financial transactions.

If the rate is reasonable then it may make sense to try to save fast to get to the 3.5% down payment on a double or triple - househacking may allow you to increase your savings rate and pay down that loan even faster.As for estimating costs, closing in NY factor at least 1% of the mortgage amount for tax, plus $1500-2000 for lawyer, title and fees, then the amount required by the bank for escrow will depend on the actual property taxes and insurance.

You can do a subject to transaction but that is not legally assuming the loan.the banks got rid of assumable mortgage long time ago. 

Some are because there are serious issues with the property that can't get financed by a bank.

The website 51west.properties also scores low on scam advisor ("owner is using a service to hide their identity, high number of suspicious websites on the server, registrar has a high % of spammers and fraud sites, website is (very) young, website appears to be hosted by a company with a bad reputation", all according to https://www.scamadviser.com/check-website/51west.properties...)Trust your instincts. 

This is also the best thing, because many out of state investors find it confusing and stay away, which keeps our returns much higher and wards off Wall Street investors from buying up swaths of houses, which we've seen in other markets.As an STR specialist, keeping up with zoning and regulation statewide is a huge part of my job.I also host monthly short term rental events for local investors, monthly webinars, and I also often get furnished, turnkey off-market STR properties.