Ughh I need some help

I am so completely cell phone Stupid Yes I admit it.
My cell is with Wind Mobile I have a Blackberry Bold.
I am in Florida from July 1-15 and I am at Disney July 5-12. I will have Wifi access at the Disney resort (although I've heard it can be sketchy) and wifi at the house in Sarasota the other days.
1. The only calling I really will need to do is 1 call for a few minutes (less than 5) once a day at Disney or at the resort.
2. I would like to be able to send a couple bbm's or txts if needed, which if I am hooked up to wifi it is my understanding i can do for free right? Or is there roaming ontop of being connected to wifi?
3. Since we are going July when it is oh so busy. I would really like to be able to use a lines app or another helpful app to help TRY to cut down on our times.
4. I do not want a HUGE surprise Bill. I know that the bill will obviously be higer with roaming charges etc... I will turn my roaming off when not needed. Turning on only if I need to use an App or make a call. I just don't know how much Data, using an App etc... costs. My rate is $1/MB. Can anyone help me with telling me how much it could cost to use the app?
5. any other information that I should know from my provider or just general tips?
I know there is the option of purchasing a pay as you go, but since my phone calls are so limited and I will probably purchase a calling card to use the resort phone when we are there to call within Florida (never to Canada) I'm not sure that purchasing a pay as you go phone is the way to go for me.
I can connect to wifi, and I can eliminate the automatic downloading of emails to help keep my bill down...
I assume with Pay as you go phones you can't access internet/line apps?
Would just appreciate some advice/experience
Thanks
Erin