Messaging should never stop working as it's unlimited with that plan. The only way it will stop is if you have it set to deduct the add-on cost from your balance and you don't have enough balance. In that case you would probably get a warning text from Telus.
I'd recommend you set up the data/messaging add-on cost to be charged monthly to your credit card. You can also set up minimum balance top ups, so that you're automatically billed for a top up whenever your balance reaches below a certain point, that way you'll always have enough.
I'm not sure about data overage, since they don't list any pay-per use data for prepaid. I'd like to say it will probably just stop, but I think it's more likely that it will just continue at some obscenely high rate. If you're lucky it's the same $.50/MB of the 100MB monthly plan, but I wouldn't be surprised if it's the same $3/MB for first 5 MB and $1/MB for each additional MB (with billing by the MB, not KB) as monthly pay-per use.
Of course it could be worse, it could be the flat rate $30/MB they charge BlackBerry users...