First time snake owner, advice needed!

[Jqualls]

Hey everyone,
My wife has been wanting a snake for years so this Christmas I decided I am going to get her one as a gift. Neither of us have ever owned snakes before and I'm not sure what would be the best type for "beginners" if you will. I have basically everything I need to up keep one (we used to have frogs that I gave to my nephew so I have most of the housing stuff) I feel like I've really done my research on how to upkeep them but I'm confused about the difference between Corn, Milk, Rat, & Garter snakes. They all seem like they can be treated the same as far as caring for them? How are they different behaviorally? I couldn't find anything regarding that online and the guy at the pet store didn't seem to know much at all.

Anyway, I know she is going to want the snake we get to be really vibrant in color which I have seen mostly in Corn Snakes (I.E Sunburst and Devils Garden) but the size does concern me a little. Do they really get to be 3 feet long?? How big of a tank would you need for a snake that size? and I've read that garter snakes tend to be only a foot long in size usually but I haven't seen any really bright colors in snakes like that so does anyone know of any that I'm just not seeing??

and lastly, my wife said something about snakes that feed off crickets but throughout my research they seem to all feed off mice? Even the little baby hatchlings eat pinkie mice so does a snake like she's talking about
even exist? I've also read that a diet consisting of only mice is bad for snakes because they need variety? So what else could you feed them instead?
Sorry for all the questions but I'm just trying to be responsible and I would feel terrible committing to a pet that I couldn't keep up with so any feed back would be greatly appreciated! Thanks!