Previously I have purchased my properties with traditional financing 20% down.  

It's usually used when the buyer can't afford a down payment or can't secure traditional financing.The advantage to the seller is they receive a steady income stream at a higher rate than if their money were sitting in a bank, assuming the buyer doesn't default, of course.I believe there are several people on BP who are very active in seller financing and feel that's it's more lucrative than investing in rentals.

Unless you plan to live in one of the units, in which case you have to deduct the rent of that unit from the income, you will need to put 25% down for any traditional financing.

Let's say one has savings in both their traditional IRA and mutual/index funds.

Bought #1 in 2004 as a home that became a rental - traditional mortgage#2 in 2012 - traditional mortgage#3 in 2014 - traditional mortgage#4 in 2015 - traditional mortgageLooking to buy #5 in 2-3 months - this will be a combo of 401k loan for the down payment and traditional mortgageOur purchases have sped up as I learned more, got more comfortable and got more income coming in to support it all.

My biggest issue is understanding how a 100% financed deal on a buy and hold multi family investment where 25% of the capital is borrowed at 10% and 75% is borrowed through a traditional mortgage could grow into something bigger.With low but positive cash flow properties we would neither generate enough to pay back the principal on the private money in the short term, nor would our investors want a 30 year loan term. 

The answer is no, not traditionally.

You could refinance to a traditional loan and hold the property after the rehab or simply sell the property if you're working on a flip.

As for resale, the value is officially determined by traditional comps but clients that are looking for a VRBO property definitely look at the revenue history and expect to pay a premium for a well-performing property.

Even if there were an aftermarket product to convert traditionally vented (i.e., vertical) water heaters to direct vent, it's likely to void any warranty and create a potential liability.