Roof, HVAC, plumbing, electrical, or just age of the homes in general.  

The original PM (now fired) didn't take a security deposit from them and with all expenses, water and electrical bills etc. they have left me with an outstanding debt of around 2200$.

It's worth getting into a relationship with a contractor who is willing to come along and do some detailed work about kitchen bathroom, plumbing electrical, foundation, roof, etc, and a number to go along with each in exchange for hopefully getting the job or if not, a fee for the work done from you.   

That 1, 2, 3 combo fixes 90% of little, nuance troubles and the home warranty covers appliances, hvac, electrical, plumbing, etc calls with only $75 out of pockets per event. 

@Stephen Bell - don't forget that some tenants have motorcycles and motor scooters ...

It has old electrical throughout the whole house.

It had no hot water heater, the electric wiring was so old it all had to be replaced, it had termites, the bathroom needed a complete remodel.

Things we have thought of are roofs, furnaces, plumbing, electrical etc...Is there any information out there that could help us out and are there any specific things that we can look for and ask when viewing properties.Any help would be greatly appreciated.Thanks!

On the other hand, I was looking at a 10-plex and the owner was paying for the water AND electricity (aka... all-bills paid). 

You are doing a very heavy rehab and replacing virtually every major system (e.g. roof, electric, plumbing, etc.), so whatever budget you come up with on your itemized budget, I would add a contingency of at least 10% (which convenient adds to about $3k) - doing a project of that scope & a property which has been vacant will certainly yield some unexpected issues (e.g. sewer lines, foundation, termites/pests).