Found a 4-plex in residential area (population <3,000) about 45 minutes away.Purchase Price: $92,000 (MLS) Bank Appraisal: $99,000 (appraised "sale" value)Mortgage: 20% Down, 5/15 ARM (self-employed, first mortgage)Rent Roll: Full occupancy = $2,050/mo; currently = $1,630/mo (3 units with long-term tenants, studio unit vacant)Roof: No leaks but needs shingles in 1-3 yearsHVAC: One gas boiler, hydronic/radiant heat and hot water; no ACOther: 2500 square feet. 

The second property I bought, I knew it needed a new roof, but I was trying to put it off as long as possible.. multiple layers of shingles and some missing.. blah, blah, blah..

For example:ROOF - Asphalt shingles should last 20-25 years.HVAC - I usually plan for a lifespan of 15-20 years, but your dollar amount looks accurate. 

When MLS first became computerized - it only worked on Windows based machines, so I grudgingly switched to a Windows machine. 

Current condition: The Good - Shingles and kitchen are about 9 years old and in great shapeNeeds: Foundation needs bracing, mortor work, drain tile, (basement is not tall enough to be considered living space, any ideas?

The shop that is on the property is at least a 20x50 shop (this is a BIG shop) and the roof has shingles and wood siding.

You probably shouldn't be ripping off shingles to replace roof decking as your first DIY job. 

Definitely cheap vs shingles!

So I'm looking at a rental online and it has shingle siding.

Between the two of you you should have a reliable roofing contractor to evaluate the roof on a full gut or just a re-shingling.