I would urge you choose Madrid or a northern locale for an immersion program. My Spanish is not fluent, but I can safelty say its fairly advanced and I have to listen very attently to most speakers from Andalucia or from Extremdura. The drop final consanants, throughout "d" in participle and in gerunds- (e.g. hablando sounds like "hablano"), seseo and ceceo are common (other threads deal with this pheomena), and the silent "h" in Castillian at times has a breathy sound at the beginning of words (e.g. hola should sound like "ola", but for many Andaluces it sounds like "jOLA". Most of my neighbors call their pronunciation "cerrero", as it is incomplete and closed sounding.
Despite this strong regional pronuncation, Andalucia is beautiful with warm people. I hope this suggestion is not taken as an attack upon Andalucia, as it is an important and special part of Spain.
Regads,
Shawn