500

Thank you for the non-zoomed in photos. I would get a Aprilaire Model 500, and install it on the right side of the return next to the condensate pump (keep drain fitting above pump, so install it up as far as you can). Then run the 6" bypass duct up the side and cut it in at the top right of your supply plenum. Put the control on the front left of the return plenum. Pipe to hot water. I think I see a water heater with PEX in the background, so you can use a SharkBite Tee Stop Valve to easily add the 1/4" supply line. Or tee it off and do a straight stop (depends on plumbing skills). A fan powered unit just isn't going to fit in this location. The combination of piping to hot water and using the digital control with blower activation should be sufficient with the Model 500 in your apartment. I strongly recommend installing the outdoor temp sensor if you can manage it.

Digital so you have blower activation and outdoor temperature sensor. The manual controls are sort of like the stone ages to me.

That is larger house, so you'll need an AprilAire 600 or 700 series humidifier. The type you get also depends on how much space you have on your furnace supply and return ducts. A fan-powered on the supply duct has a bit more output than a bypass humidifier. I would not get the Model 500, that is too small. The 500 is meant for apartments, condos, and small houses. The 600 is the larger size of basically the same thing. The 700 adds a fan for additional airflow.

FYI the Lennox humidifiers are rebranded AprilAire humidifiers. The HCWB3-12A is like the model 500 and the HCWB3-17A is like the model 600. The 17 is the gallon per day output of humidity evaporated, not how much water it uses. They will both use 72 gallons of water per day if they ran for 24 hours nonstop. If you figure the humidifier runs half the time, the water usage would be around 1000 gallons per month. Try putting plastic on all the upstairs windows to reduce air leakage and humidity loss.

If you own your own home, get the AprilAire that attaches to your furnace. Idk how much they cost but it would be cheapest in the long run.

If you want whole-house coverage, Aprilaire units connect to your HVAC and are the gold standard for set-it-and-forget-it. No daily maintenance, just annual checkups.

I have a whole house humidifier on my HVAC and it's struggling. It's been incredibly dry lately.

We have an Aprilaire unit attached to the HVAC trunk in our utility room. We shut it off as soon as it gets to the Springtime so no issues with mold. 

I had Eric from Valin’s heating and air install for me a AprilAire version and honestly it was the best thing I did! He explained to me all my options, it really really made a difference in my house. If you need his phone and wanted to ask 703-239-3633, Valinshvac.com. Hope that can help

Aprilaire makes solid dependable equipment. Had one for 3 years w/o issue

If you have central heating and air I HIGHLY recommend looking into a central humidifier especially if you are in a very dry area. Set and forget with auto water feeding and humidity setting with minimal (like 1x a year) maintenance. They are usually not super expensive to install either. I like the Aprilaire units.