The part where I'm not clear on is for an existing rental where tax returns are available.I know that Principal, Interest, and taxes are all included as debt.  

I had a quote of 2.64% for a 4 year term on my principal residence in Canada just today.The US starts around 4% but then you have to add 30 to 40% for the exchange if I use Canadian money for the payments.So, ideally I need to borrow as much as possible for my deals and pay it off as the dollar improves. 

From the mysolo401k website: If a participant wants a repayment period longer than five years, plan administrators should obtain a sworn statement from the participant certifying that the loan is to be used to purchase the participants principal place of residence (a principal residence, has the same meaning as the term under IRC Sec. 121).  

You can also try to find some creative solution to bail yourself out from your current condo faster - I have a friend that rents out all the bedrooms in his place and uses that money to pay down the additional principal. 

Is it worth it to do this for just principal paydown + appreciation with only moderate leverage?

I have contracts and forms in my fileplace, they are basic boiler templatesIf you're doing lease option assignments please get a license in Alaska and let people know you're acting as a principal and not as an agent@Tricia O'Brien

First, he doesn't even know how to spell principal (so probably doesn't understand the concept). 

He has extensive experience in commercial loan originations, including his position as principal and mortgage broker for CHF Real Estate Corp., where he originated commercial mortgages across all asset classes.

One thing to be careful of thing is that some lenders require a minimum amount for any principal prepayment, in order to re-amortize your loan and adjust the P&I payments downward to reflect your principal prepayment.

Some have better splits but you get the idea.Me I negotiate for 2% or better on the buyer side and since I am one person and the principal broker I keep the whole 200k plus.So 2 schools of thought.