I am working on a similar project. I found that Rogers had a plan that worked for me. I bought a SIM card from them for $9.99 plus tax. I then bought a $10 "Pay as you go" voucher. For the plan, I got a "Pay by the Minute" plan. To that, I added a Text Messaging Pack.
I chose the "Pay by the Minute" plan because it doesn't have a set monthly fee. It just charges $0.40 a minute for phone calls. Since I'm not making any voice phone calls, I don't pay any $0.40 charges. What I do pay for is the text messaging.
With the Text Messaging Pack, they have some flat rates for texts. For $5, you get 250 texts. For $10, you get 2500 texts. The numbers are for 30 days. The only down side is that if you don't use up all you texts in that 30 days they don't carry over. You can get a plan where you pay $100 and you don't have that problem of the 30 day carry-over. Each month, they just automatically draw from the $100 for your $5 or $10. So you could get, for example 2500 texts a month for 10 months for your $100. I hope that makes sense.
So for my project, I decided to go with the $10 voucher with 2500 texts for 30 days while I am testing. If all goes well, I will go for the $100 plan for the long term.
I hope that helps.