Well, you should remove the Spanish text before you typeset your English translation. Don't put one on top of the other. It'll look terrible. (Not sure if that's what you were planning on doing, since you didn't specify.)
Also, from my experience, it's better to translate into a text file first, then typeset the translation back.
Going bubble by bubble to translate, remove previous version, and typeset the translation will take much more time and the translation might not come together too well as a whole.
As opposed to what? Starting from raws (Japanese, Korean, Chinese)?
In most cases, using Edited Mangas will be easier since they should be cleaned already (unless, the group did a bad job). But if you're planning on using somebody's releases, you should ask their permission. So make sure to do that first.