Cobham is very nice, and quite pricey. Consider Woking, much cheaper (also cheaper than Guildford) and 25 mins into london with trains every 10 minutes or so. Woking to Cobham would be a nice 15 min ride, it's fairly easy to make that ride almost entirely off road too.
Tons of good riding around, ash ranges is full of singletrack, leith hill is nearby, and close enough to the M4 you can get to Cwmcarn in 2.5hours without flooring it. I live in woking, and the thing I like best is the fact I can run/bike out of my door and be off road in a couple of minutes in some pretty decent train.
Worth pointing out that Woking / Guildford / Cobham are pretty leafy bits of surrey, the further in you get the more Clockwork Orange it gets. Personally I'd avoid Dorking, don't really like it compared to Woking / Guildford.
Be worth checking with someone in the area which roads are rubbish in the mornings. If you end up near Woking, give me a mail and i'll give you the low down (ewan at ewanpanter dot co dot uk)