100k is probably the limit to about any spark plugs. I mean, I did get more than 150k out of my 2002 4Runner and she was missing like crazy because of it. I won't go as long again. Which reminds me...I'm at 260k. I better go check them.
For the Mazda 5, you're due. I would only replace them with OEM. I haven't done it yet so I can't kabitz on how easy/hard it is. But I'm pretty sure my 2010 has all of them on top under the plastic cover.
Get a 5/8 (16mm) spark plug deep well socket. Nearly all plugs are this size. They are special and come with a rubber insert inside so the plug doesn't fall out or get damaged. You will need an extension too.
Get dielectric grease and smear that stuff in the boots.
I do not know the Torque specs for these. You should Torque them properly. As a reference my 6cyl 4Runner is about 15ft-lbs I believe.
When putting plugs in, never use your ratchet at first. Always just by hand with your extension to get it started. A cross threaded plug will not be a fun time.
-Mike
Edit: I originally said to replace the wires too...but looking around...I guess not for this engine?