Mixed Use Investing - Residential and Commercial

[Joseph Hoot]

Although I'm very new to real estate investing, I've been reading a ton of material and taking notes along the way.  One thing I haven't dealt with is any reading on mixed use property.

I was looking at 601 Fulton St., Buffalo, NY as a potential investment.  Since that time, it looks like it sold for $23,000.  They originally started listing it last year for ~$48,000.  It looks like it needs a lot of new windows, some work inside, and probably a new garage door over the next year or two.  So $23k was probably the right price.

It's two buildings that are 2-units each + an extra area at the front that was once a store front.

In this situation, we have 4-units = residential between two buildings.  I think I understand everything that I need to deal with that.  But I've never dealt with understanding what it would mean with mortgage terms and such when dealing with a potential commercial unit.  

Would someone mind helping me understand what I might expect to change once that fifth unit is available on the property?   If I were to have purchased these two buildings and started just renting out the 4-units, then later wanted to lease out a commercial unit, how might that affect my loans?  Are there any other things I would be concerned about?   Does anyone have any recommended reading with regards to mixed use investments?

Thanks,

Joe