Thank you.
Will refer you to the following link regarding VHF vs UHF.
http://www.shure.com/americas/support/find-an-answer/what-is-the-difference-between-uhf-and-vhf-frequencies
Other factors may be involved and you can easily do some additional research regarding options and trade-offs before choosing.
As for amp, etc. itself, I agree that a single unit would offer portability and simplicity.
However, a single unit will put "all of your eggs in the proverbial basket". And sooner or later there will be some requirement that a single unit will not be able to fulfill. You will end up needing another component that may not be configurable with Rockville Amplifier/Mixer.
Not familiar with that product at all. Only 18 reviews and six reviews are 3-star or less per Amazon so I would be skittish about the quality as well as its suitability overall. Aside from being a reference point for requirements and specifications.
Another concern is the existing ceiling speakers: what impedance are those speakers and how are they configured (series, parallel)? The required amplifier/mixer must match accordingly.
Not able to directly suggest a particular product.
Since you have provided some additional information someone else may be able to suggest a suitable amplifier/mixer or otherwise add further comment.
There are some very knowledgeable audiophiles within this forum. Hopefully one of them will post accordingly.
One question to anticipate: what is your budget?