If you are already used to playing with a click like that, then a pretty simple solution would be a small mixer like this one
. You can put the monitor signal from the board into one channel and the click into the other.
FYI, I suggested that board because it has a built-in compressor on the first few channels. This acts as a limiter so feedback or sudden "Are you ready to rock, Springfield!!"
lead singer screams won't blow out your ears.
Another option is the Tasty Blender
. This is one that is more customized for guitarists and drummers. I have the first-gen model that is just a single input, but this one has multiple inputs. It also has a limiter.
For the actual in-ear monitors, I would recommend a set of the Shure SE215 or equivalent.
In-ears really help with making the monitor mix more consistent and once you get used to hearing everything at a nice comfortably lower volume you won't miss the "amp headache" the next day after playing.
You can get a "closed" off feeling from the rest of the band or the audience. Getting a good monitor mix is key because you can't just "lean over to the right in order to hear a little more guitar during the solo" and you can't adjust your dynamics off of the rest of the band.
Overall, affordable and good quality in-ears are a good investment and will help you in the long run. I wish that I had them 20 years ago.