You can say "we just want to come take a look around, make sure there are no pipes leaking, no moisture from condensation in bathrooms or around windows, and that everything is in good working order for your use, and our eventual sale of the house" (if they already know your plans).Especially if you have any suspicions..

Our house was built in 1993 and has has it's original HVAC system (3 condensers and 3 evaporator/furnace/air handlers). 

You can use it loosely to compare to investment options, but without valid, realistic projections before the purchase, you investment will not have a chance to perform anywhere near what you projected it to.My property that I described above is not a good investment because it has cash flowed so well on actual numbers anymore than it will turn into a bad investment if I get hit with a $2500 repair bill one month due to a sewer line replacement, or an insurance claim on the roof, or a new ac condenser.  

I would suggest that when you go through the property you can look for condensation on any exposed water lines (sometimes you can see on the toilet tank or in the basement) if there is water condensation that seems to indicate there is a slow leak somewhere.You could also call the local water authority and ask them what an average bill is for a 2 bedroom, etc. and they may give you some good info.Good luck!

Is it rusty because some water dripped on it or the condensate line dripped on it or is it rusty because it is 15 years old.  

Understanding that there is a seller's disclosure, but we check to see how old the A/C unit is, if it's still on R22, we ask if both the condenser and air handler was changed at the same time, we walk to see if there's any foundation issues, and look at the roof.

It's definitely possible to do better than the numbers above - you just don't want to count on it.To condense everything above into my gut reaction - risky, definitely potential for headaches, but potentially profitable.Hope this helps.

I like BP.com because it's the very definition of education: condensed experience.  

Meaning filth, condenser hit be the lawn mower,  not mowing, plumbing not working correctly. 

My property management company is quoting me "14 sear 410 A freon 4 ton Goodman complete system they would be looking at $5500 That is the condenser evaporator and furnace."