@NellytheEl I moved to Plumstead a few years ago. We're really happy here. There's lots of positives about Plumstead. There is loads of green space. For some reason, the green areas don't all show up as green on Google maps. As well as winns common and Oxleas wood that show up as green, there is also Plumstead common and Bostall Wood. It's amazing to have these green spaces on your doorstep. Shrewsbury Park is really lovely and has amazing views over the Thames.
The transport is good. The bus routes are regular and take you to woolwich in minutes, where you've got the reliable dlr (and one day, Crossrail). The trains from Plumstead train station are regular and reliable. I'm surprised about how little I miss the tube- it's much more pleasant commuting to work on an overground train compared to an underground tube. At rush hour, they are never crazy busy like the tubes can get.
There are some good amenities around. Two lovely cafes in Plumstead. Some nice pubs including the Plum Tree and The Star (I love the sourdough pizzas at The Star). There's some amazing south Asian restaurants. In Woolwich, there's some lovely cafes. Boulangerie Jade is one of my favourite places in the whole of London.
Plumstead also has a really good community. There's loads of community activities and I've got to know my neighbours, which is completely different to where I used to live.
I'm not going to pretend it's a fancy, affluent area. The high street is always full of litter, which annoys me. But it feels no better or worse than most areas of London. Of course if feels different to the affluent areas, but if you could afford to live in such areas, you wouldn't be considering Plumstead and Newham 
"Upper" Plumstead is definitely more pleasant than "lower" Plumstead. Lower Plumstead is the area around the high st, particularly north of the high st. Once you go up the hill and head towards the commons, it feels more pleasant.
There are a few houses in upper Plumstead in your price range. I'd choose a house in Plumstead over a flat elsewhere. Having moved from a flat, I love having my own front door and not having to worry about communal areas etc.