Welcome Jane! You're going to loooove this hobby. Trust me on this ;) Only a little later on down the line you will probably refer to it as your "addiction", "passion", "craft", "artform" etc. :lol:
I've never used an ironing mat with a grid on it so I can't help you there. Surely there will be someone on here who has one and can tell you whether it's worth the money or not.
It can be a bit expensive to get started, but your tools should last you quite a long time. Your fabric won't. It needs feeding every couple of weeks at LEAST!! LOL
Hope you enjoy the site Jane. :)