I got one of these today, replacing an old iPhone, and I have some questions

The first one, is relating to sending text messages. On my old iPhone, if I didn't have mobile reception (and I rarely do where I live) then the texts went via WIFI as an iMessage - is it possible to do that with the S5, and if so how? Without using an app I mean, I have WhatsApp but not all my friends use it.

That will do for starters

Imessage is only useful to text other people with Iphones. Hangouts can send sms text and messages via data but it does require the recipient to also have Hangouts to receive messages via data. So it's not as simple as imessage but if you struggle for coverage at home then maybe it's best to look at other networks.