Cycling to Catford (along the path by the Pool River) could also be a pleasant route to work if it is not raining. I'm not sure what the provision of cycle racks are like at Catford, but there are plenty at Forest Hill. However, there is a slight incline on the route to Forest Hill whereas the river route is completely flat. (Not that anybody should take advice from me on riding a bike, I haven't done it since I moved back to Forest Hill - 15 years ago).

It probably depends where you work, but a season ticket from zone 4 (Lower Sydenham) to a London terminus might be cheaper than a travelcard to enable you to travel on the buses as well as trains. It just depends if you need buses/tube beyond your London terminus (if you don't work within walking distance of London Bridge, Charing Cross, Waterloo, Blackfriars, Canon Street).

Also you have to weigh up the additional cost of the zone 4 ticket compared to your additional mortgage/rental payments. Chances are that the difference in housing costs more than covers the extra required for zone 4 ticket, so you are still winning by moving close to Lower Sydenham and getting a zone 4 ticket.

I think you would find that it is quicker walking to Catford rather than taking the bus, due to the traffic on Catford Hill. I agree with blushingsnail that the walk would probably be quicker to Sydenham (or to Lower Sydenham, which is the closest station to Bell Green). One day in the future we will get Lower Sydenham station moved to Bell Green, but we might all be retired/dead before that happens.