leeching off a public wifi will be less reliable, probably significantly slower, and possibly subject to (unknown to you) use limits of their own.
pretty sure your provider has an unlimited option for a little more per-month. just upgrade to that for awhile, then go back to the capped plan when you're no longer going over its limit.
if you need to stay within a provider's monthly limit, just set 'speed limits' on your downloads so you won't go over for a month, and also so you have some bandwidth, and quota, leftover for other online 'activities'.