Two seniors (almost 70 and 80) suffering from information overload while trying to pick a mid-level all-inclusive resort in Playa del Carmen in late January! Have never been there, and have not done all-inclusive but want to try. Hope someone can suggest one or two resorts with good food and relaxed atmosphere in the $350-$450 per night category. Thank you!

I cannot as I only know the "other side" of Mexico for resorts (Puerto Vallarta). I invited you to visit the Trip Advisor forum, similar to this one in format, but way more activity on the Mexico branches there. You can read without joining. Or join and ask your questions. That is where all my best information comes from. Not too many people on this branch at Fodor's with the information you are seeking.
buena suerte, suze

Do you want an adults only resort?
Not sure if the price will fit, but look into Royal Hideaway Playacar. It is an adults only AI on a great beach. Very laid back atmosphere.
Also look into Catalonia Royal Tulum which is also a laid back adults only on a great beach and might be less expensive.

I've been to Playa and Cozumel in January, but don't know the AI resorts. It's obviously a popular time of year with snowbirds so don't know if you'll need to bump that rate up a little for AI.

Google Maps now has a very good search feature for hotels, successfully taking market share away from TripAdvisor because they don't try to sell you stuff.

Playa Del Carmen is known more for smaller boutique hotels than big resorts, though are a few in town. Most of the all inclusives are between the airport and town, if I remember right. It’s been a few years since I was in Playa.