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Any suggestions on whom to approach?

I suppose by the description of what you are looking for then a main dealer of the brand you want.

Do dealerships offer "Authorized Used Cars"?

Yes

Also, our experience is fluency in Spanish (which we are not) is required to deal with these dealerships (New and Used).

Take along a friend who is fluent or at least fairly fluent. Never sign anything unless you are clear about what you are purchasing.
There will no doubt be English speaking dealers dotted around. But will they sell new or KM0 cars?
Do a search for KM0 {Kilometre zero}

Buying a car was one of the trickier things we did when we arrived in Valencia. The dealers didn’t appear to take our emails seriously. Maybe they think you will not have NIE.

I get the feeling you have not found out about the way Spanish like to undertake business. Face to face is always the way I have found works  very well.

RibeiraSacra wrote:

I get the feeling you have not found out about the way Spanish like to undertake business. Face to face is always the way I have found works  very well.

Yes, I completely agree. People think you will not buy from them unless there is a relationship. Very different from northern Europe or the USA.