Caceres

Caceres is about 185 miles from Madrid. The Renfe website (renfe.com) isn't working for me right now, but I think the travel time by train is between 3 and 4 hours, which isn't bad for the distance being covered. It isn't in any case a day trip, because there's too much to see in Caceres not to spend some nights there.

I don't know anything about multi-day tours to that area.

My visit to Cáceres was in 2006, so I'm not exactly up-to-date, but I recall it being a great city to see. I agree with acraven; it deserves at least two nights, and possibly more (I think I stayed four) -- especially if (as I recommend) you also visit the historic town of Trujillo, a modest bus ride from Cáceres. And yes, I would recommend going to Cáceres by train.
I would never discourage anyone from hiring a tour guide; many local people make good livings as tour guides, and hiring one is a good way to put a little more money into the local economy. But at the same time you may not really need one for Cáceres -- a good recent Spain guidebook may be sufficient, though maybe there are a few little secret things about Cáceres (or any city) that only a good guide will know.

Like my response to your Mérida query. I don’t see Cáceres as a day trip from Madrid. If you had sufficient nights to stay in Cáceres, Mérida is only an hour from it.