I like it so far. But you got alotof work ahead...
Word jumping does not look good...,to avoid them you can check the height of the textfield, when you add the whole word:
msg - original message
m - message textfield (variable)
mm - message textfield (instance name)

if (msg.charAt(n)==' ') {
s=msg.slice(n+1).split(" ",1);
th=m.textHeight;
l=mm.length;
mm=mm+' '+s;
if (th<m.textHeight) {mm = mm.substr(0,l)+'\n';n++;k=0}
else {mm = mm.substr(0,l)}
}
mm = mm add msg.charAt(n);