When to replace A/C & Furnace?

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Hi all! I'm new to BP and was hoping to get some advice. I have a 25 year old single family home with its original A/C & furnace. The A/C unit is pretty rusted outside but still functions. However, my tenant notified me that there is water leaking from the humidifier and drain so there is a small puddle underneath the furnace. Most likely the drain pipe is clogged which I can service or take care of myself but the repair could be more expensive.

In November 2014 the furnace had a minor issue with the furnace thermo coupler which cost less than $80 to repair.  The furnace is also 25 years old. 

Should I replace the furnace and A/C to reduce future problems or keep repairing & servicing so I don't have to make such a big upgrade right now (tenants are planning on staying here for 2 more years)?