Just bought a used Pen EP5. How careful do I have to be when using it?

Olympus cameras generally are made to take some abuse, but you do want to be sensible about things like rain, dropping the camera, etc.

Note, since the E-P5 was announced in 2013, and Olympus generally only makes cameras in one batch before moving on to the next camera, that means your batteries are probably 4 years old at this point.  You might want to start thinking about buying new batteries, as 3-4 years is when lithium ion batteries start losing capacity.

Now in terms of batteries, Olympus changed the BLS battery over its lifetime.  The original BLS battery and charger had to be redesigned to meet a new law in Japan (where chargers turn themselves off once the battery is charged).  New BLS-5/BLS-50 batteries are gray, and the charger is gray.  Old BLS-1 batteries are black and the charger is black (the current BLS-50 is like the BLS-5, except it has more capacity).  You can use either the BLS-1 or BLS-5/BLS-50 battery in the camera, but you can only charge BLS-1 batteries in the BCM-1 charger, and BLS-5/BLS-50 batteries in the BCM-5 charger.  I think 2013 is around the period after Olympus had switched batteries but had to switch back in non-Japan countries because an earthquake knocked out their battery production facility.

That means if you have the black charger, and you want to get a new Olympus battery, you will probably need to pick up a new charger as well.

There are third party batteries that are much cheaper than Olympus's batteries.  A lot of these batteries are junk, but there are some good ones.  I used to like Wasabi batteries, and my Wasabi BLS-5 clone batteries lasted for a few years.  But my newer BLN-1 clone batteries from Wasabi had to be replaced within a year of purchase, and my older BLS-5 clone batteries also have had to be replaced.  But perhaps there are other brands that people like.  I use Watson batteries now, but you can only find them in a few places like B&H, and you would want to find a local supplier to cut down on shipping charges.