How good/bad your commute is depends on where you are going to live, and where inside the M25 you want to go.
For example, I live north of London in Bedfordshire, and I work freelance, so I'm not at the same place every time I go into London. The train that goes through my town goes directly into Kings Cross station, so when I work in Farringdon (kinda central London) the commute is easy - I get on a train at 7:55 and I'm at work about 50 minutes later.
When I work in Gunnersbury (southwest London) it's a different story. Taking the train to Kings Cross takes the same amount of time, but the tube across town to Gunnersbury takes another 40 minutes. The whole thing can take 2 hours depending on whether or not everything runs smoothly, and between weather, broken trains, strikes, people falling/jumping on the tracks, and people stealing the electrical wires from above the tracks, it's not uncommon for it to not run smoothly.
So when I'm working in Gunnersbury I drive, which takes an hour if you ask Google Maps, but that would be under optimal conditions - 2:00am on a Wednesday morning when they're not doing any road construction and it was sunny and warm the day before. Tomorrow I need to be there at 8:00am, so I'll be leaving at 6:00am.
But here's the catch: If I lived somewhere southwest of London like Woking, the journey to Gunnersbury would only take 30 minutes or so by train, and the journey to Farringdon would be crazy long.
So if I were in your shoes, what I'd do is look at where in London you want to end up when you're commuting in, and work backward from there. Find the closest main rail station to your end point, and then figure out what rail line goes into it, and look at towns along that rail line heading out of London. When you get to one that's nice and affordable, live there.