Obviously it's entirely up to you as to whether you do your CELTA full-time or part-time; it makes no difference to the qualification that you get at the end of it. Personally, I would go for the full time course. From what is supposed to be quite a stressful experience, I think it's better to get it over and done with in a month, rather than being stressed out for three months! I also think there's something to be said for the pressure actually bringing the best out in you. But it also depends how you learn - if you think you would be better learning at a slower pace, then go for the part time option. Go for the option that suits you best.
Volunteering is an excellent way to see if it's for you - if you have the time, try and get as much experience as you can. I worked in a primary school helping non-native speakers with their reading. Any additional experience you can grab is definitely worth it, considering there is a rather growing glut of CELTA graduates with no experience.