So, if I happen to get the original property and still want to pursue this short sale, I'll have to find a way to fund potentially two properties at once, when I only really have a down payment for one.

I think you want to be really clear of what is owed versus what the home is worth before you sign up for that.If its still something you want to pursue - there are almost no assumable mortgages anymore, however if he quit claims the property to you and you keep the payments current, it is not likely the lender will do anything to object to this.

Currently renting out the old primary residence at $1800/month with a cashflow (after mortgage, insurance, taxes, vacancy, maintenance, cap ex, etc.) of over $500/month; with the other two investments each pulling in between $250-350 a month.I also decided to make tennis my "full-time" job and pursue real estate more heavily as my career.

My question is that I am looking for another avenue to pursue for lead generation and thought about NOD recipients.  

Currently, I invest in SFs and location is very important for any prospective properties - 1) because it may attract a certain tenant (ie. great school district, or close to a college campus) and may also help me avoid certain tenants that might be troublesome and 2) future resale/appreciation.I am interested in pursuing MFs, and I find some really great cash flowing properties in my city (and out of state), but they are located on the edge of what might be considered less desirable part(s) of town.  

Question: I would like to use equity from our home to pursue a second investment.

Any leads on reputable lenders that would pursue this loan?

Now that I know real estate investing is something I'm going to pursue, I'd like to use this property as a foot in the door to REI. 

@Mark SpritzWhile in the the year that you open the solo 401k plan you need to be merely pursuing self-employment activity (see IRS Publication 560 which lists this specific language), it is important to note that generally in order to continue with the solo 401k plan you must continue to to perform "material services" with the intent to generate earned income from self-employment.

I'm an advocate that you can't buy anything off a calculator since they are often so co servatice they may prevent you from pursuing a good deal so getting into as many multi family properties shortens your learning curve- make sure you have a realtor that is a pro on multi family properties or owns some as they would be invaluable