My husband and I are literally just doing the same thing due the birth of our son!. We have come across a lovely little place called Flitwick in Bedfordshire. It is on the Thameslink line and so is 35min to kingscross and 45min to city (this also means if you work close to these areas then you will not have to pay for an underground ticket as well). In terms of road access it is just off the M1 (J12). It is close enough to Luton, Milton Keynes to make use of their shops, yet far enough away not to be anythin like them!!
It has its own web site you can check out and if you go to upmystreet and put in mk45 you wiil see that it's acorn profile 13 which is pretty good. It's house prices are above average and in demand due to it being a decent place with decent schools! However they are still cheaper than London and if you are prepared to put in the work and go up to view in the evenings you can still get a nice property before they are snaped up by people closer.
Other decent places on the Thameslink line are Westoning and Harlington. There are also plenty of other nice places a short car ride from these stations such as Ampthill and Harpenden.
We tried looking south i.e. kent but the journey in was too hard and expensive (rail card and underground ticket, due to the trains not coming right in to the city).
Sorry to go on but I have really fallen for the place, it's surrounded by fields and forests, yet has good road/rail access,it's own well equiped lesuire centre (which does mother and baby swimming and has a creche),great shoping near by and has it's own 24 hour Tesco's by the station!
I'll stop now as I'm starting to sound like an advert!!!!!!!!!
Ohhh one last thing it's claim to fame is that it's leisure centre is the one from the old t.v. programme Britas Empire!!
Good luck with your search. If any one wants more info about Flitwick just let me know.
As for the journey from Brighton I have heard mixed reviews some people have said it is ok others have felt it is too long as it can be 1 1/2 hours by the time you add on travel at each end.