Borrowing other people's money sounds great but the market prices are high and hard to make profit if you borrow the down payment and borrow for the remainder of the mortgage loan. 

Let's say I have 500k down payment and borrow 500k, and the monthly mortgage payment is 5800, and out of that, 4800 is mortgage interest, and I pay 2500 annually for insurance and 10000 for property tax, could anyone please let me know how much UBIT I would be subject to?

It will be difficult but not impossible to find a lender that will take second position and very likely the first lien holder will be apprehensive to a subordination agreement..The terms will not be as favorable likely in the 7-8%'s I/O but on a new mortgage there are investors that theoretically can finance 'unlimited' units..I think with our team it's 250+ up to $100M.

As for selling with 10% down at 8% interest, you now have two title transfers that violate the due on sale clause and add complications to subject to “safeguards” that reduce risk if note is accelerated, if party in possession doesn’t pay taxes or insurance, and of triggering whatever system the lender has set up to notify them when a deed transfer takes place. 

A trust may offer some of the same benefits as an LLC, depending on your goals, and allowing you to claim the part of the house you live in as homesteaded tax.Make sure you have the right home insurance and consider an umbrella policy for extra protection.Certainly buying a house, using conventional financing with limited money down as a primary residence and renting out parts of the house to help pay the mortgage, is a smart financial move.

Try a private lender  and a Joint Venture

They all appreciate but peninsula is more stable in downturns and recovers the fastest, I witnessed this myself in 2012 further in east bay was still about 40% down and peninsula and "tech" cities already recovered fully.I am not sure you need a loan or not, recently I asked my mortgage broker he told me for a second home it would need to be at least 50 miles away and also in a city known for vacation activity .   

I am an investor and lender in the Florida market.

Typically HELOCs have adjustable rates and if the banking climate changes I believe the lender can actually cancel them at their will.

Seller financing can be a way that 1) You aren't hemorrhaging mortgage payments while paying for work to be done.