That is the standard list of documents required by all banks if you are using a traditional loan. 

You can't combine Roth and Traditional money in one account unless you do a Roth conversion, which would be a taxable event.There is no one-size-fits-all when it comes to investing using your retirement funds, but you certainly can own rentals with leverage in an IRA and this can be very profitable. 

It doesn't have to be the traditional live in one unit and rent other out multi-family.

For one thing, a traditional property manager is not likely to want to manage your property in a rent-by-room scenario.

I would like your opinion on whether it would be better than finance the whole purchase with a HELOC from my primary residence or only get the down payment from there and get a traditional mortgage for the rest. 

Assess your financial situation and consider financing options such as traditional loans, private lenders, or partnerships.

Conversely, the same two-bedroom, one-bath unit could fetch $1600-1700 under a traditional LTR arrangement without furnishing or utilities.Considering the added expenses, the MTR/STR model doesn't seem worth it for me, given the marginal increase in revenue.

I am in an area that traditionally makes guests bring EVERYTHING I just mentioned in both paragraphs above, including sheets and towels, so they are happy with what I provide.  

Traditional brokerages are also ‘Pyramid’s’, however, the agents are at the bottom!  

You may also explore a DSCR second Mortgage if you do not qualify for traditional financing.