Do i need a Wi-Fi data plan for an iphone in order to text on imessages?

I'm thinking about buying my sisters old iphone 4 and i don't want to have to buy a Wi-Fi $30/ month plan, but i will have a regular phone plan (texting/calling)
will i need to have Wi-Fi in order to text (on imessages)?

For iMessage you do not need a data plan because with imessage you can only message when you are connected to Wi-Fi. But you have text messages where if you buy a data plan you can text any where you want. If you want to phone people and don't want a data plan i suggest downloading magicjack on the app store for free calls when you are connected to Wi-Fi and it will only work when you are connected to Wi-Fi and if you want to call when you are not connected to Wi-Fi you will need a data plan.

If you are using your old phone for calling and texting and your sister's for everything else… You will need to pick up a Wi-Fi signal from somewhere to use any internet applications, but you would not need to purchase a monthly plan.
If you plan on just moving the chip from your old phone to your sister's new phone, it will work, but the phone company will automatically add a plan to your phone. The towers "sweep" every so often and will pick up that you are using a smartphone. Smartphones require a data plan.
There are apps such as text+ that allow you to text, but they are definitely not as reliable as regular texting, I have Wi-Fi at home, work, and where I live there are many hotspots in restaurants or other public places. I have my old iphone that I can check email, take pictures, etc on but keep my really old phone for calls and text so that I don't have to pay the extra $ anymore. The iphone with no chip is basically an ipod…
What I don't like is that I used to use the GPS for driving directions and can't do that without the data plan. Most anything else that I want to do will wait until I can find a Wi-Fi hotspot. It is also weird to carry 2 phones but I would rather save the money. Most people don't even notice because I keep them in my purse anyway. Occasionally if someone sees my really old phone, they will comment about how it is a dinosaur or that they didn't even know flip phones still existed… Thankfully, my identity is not tied to my possessions or other people's opinion of me, so I have no problem with their comments. I just brush it off or if they persist in teasing, I smile and say something like, "My dinosaur is allows me to put my money to better use like donating to programs that help women recover from substance abuse. What does your smartphone plan allow your money to do?"
So, I would suggest that you either pass on the phone if you don't want to pay the extra for the data plan or be willing to carry the both.