Your £700 quid rent limit could be the most critical decider! The faster the rail line to London and closer to the major A roads the more you will pay. If the property is close to these and still cheap then it will be one of the areas that previous posters have told you to steer away from. As you don’t need to commute to London you should be able to find somewhere nice. For beautiful morning walks, environment and great country pubs then any of the rural areas west and north will easily take on anything you can find in Kent. Now after saying that I’m going to contradict myself, as follows;
I only know South Essex from my experience for mountain biking I would choose Danbury, Brentwood or Leigh-on-Sea side of Southend as there is very technical hilly stuff straight from your front door. Don’t forget the Olympic Course which will be open next year.
For an easy commute into Southend coming from the M25 towards Southend is a pretty nice run compared to trying to get there from Chelmsford. As always aiming to get into work for 7:30-8:30am is much easier than leaving it later. There are usually lots of rat runs once you get to know the place. Coming from Oxford you are probably used to the traffic.
Getting into Chelmsford is always a pain but A12 is normally clear up until you get close, approaching from the north may be worse than from the south but I only do both a few times a year.
If you want to commute to both places by train then you want to be on the Liverpool Street to Shenfield line but from talking to a mate who commutes north from time to time it’s not cheaper than using your car. If you’ve not got any parking spaces at your office then you could be in for some fun and expense!