Send text using data only through an app? (including to people who don't have the same app)?

Ok so I want to buy a new phone but just want to buy a data only plan, so am I able to send and receive texts using data?
By this I mean I send a text and it shows up in their messages as if I texted them normally. For example, I'm using an app to send texts and I send a text to a friend with an iphone who doesn't have the app and he replies back and I get it in the app.

Sorry about such bad explaining.

Sounds like you want a simple texting app, I use Handcent sms.