You’re right in saying it would be good experience and not many opportunities like this come your way when you are an inexperienced teacher. I can’t give you hard and fast advice but I can give you some areas to consider when you are making your decision. In the end only you know whether it will suit you.
The first thing to think about is that if you are teaching 5 hours a day you will be working approximately double this time. Planning and preparation takes much longer than you think, especially when you are a new teacher.
You say you don’t have any TEFL qualifications or experience, so find out what sort of support will be available to you. Will the course be taking place in a language school? Will there be other teachers around and somebody to turn to for help with planning? Also consider more basic things such as access to material to use in class and use of a photocopier.
Is your Spanish good enough to teach grammar? You’ll need different language to what you use in your every day conversations. It can be tiring working all day in a language that isn’t your mother tongue.
What are the expectations of you and why are these people learning English? Do they really need to study grammar or is this decision based on what they learnt at school? It will be extremely tiring for everybody if all you do in class is grammar. You say the people are unemployed, they might benefit more from spoken English as this could help them re-enter the job market.
Is there a chance for you to share the teaching load with another person?
The final decision will be up to you. Only you know your personal circumstances and levels of challenge you are willing to accept.
If you decide to accept, I wish you the best of luck and you can always keep in touch with this forum for advice on more specific aspects.