Help choosing a new watch?

Hi there, I was wondering if you guys could offer some advice on choosing a watch. I'm starting training for a marathon/iron man and would like a watch to help with that. I want it to track my running, cycling and swimming and upload to strava, I'm using Runna for a training plan so if it could work with that too it would be great. I'm thinking of doing trail running as well so I'd like it to show OS maps or other similar colour maps on it. There's so much choice and many different prices so I'm a bit overwhelmed.