OP, I live in South Norwood. No, it's not trendy or fashionable and yes, it is rough around the edges, much like lots of the outer zones of London. If you know London then I imagine there are lots of areas that you would love to live in but, for £450k it's highly unlikely that you'll find a property that is suitable for a family in those areas .
South Norwood is nowhere near as bad as suggested. It's diverse and it's traditionally a working class area which may give some people the heebeejeebies. It also needs tidying up, a lot. But, this is happening.
You've mentioned the Country Park area - a lot of families from other parts of London have and are continuing to move here. A lot of young professional couples are buying houses here. The main road which runs past the Country Park area is currently undergoing improvements which will see tree-lined paved streets and new seating. There are lovely new shops - galleries, a haberdashery and coffee shops. There are two lovely new pubs which are definitely more current/trendy and an independent bakery about to open.
There are community groups here that are really active and want the best for the area which has led to the above. There's also a monthly market near the station, a weekly Park Run at the Country Park, a local cinema, the impressive Stanley Halls which hosts a theatre, a cafe, weekly yoga sessions and more. This has all happened because of the strong community groups.
Most importantly, for a young family, the schools are improving. Oasis Ryelands which is a Primary school in The Country Park area is a wonderful, nurturing school and has just had a good, with outstanding features rating from Ofsted. Also, the area is so green - it's surrounded by parks with lakes! Who would've thought!
I'm newish to the area and I moved here from a more central part of town so I know it can be daunting but it really is ok here.