From everything you've said, my first thought is Hatfield Peverel or surrounding villages. But I'm not sure your budget will go far there.
Colchester has always been a bit of a mixed bag, but it's undeniable that it has gone hugely downhill in the last 10 years or so. Crime, anti-social behaviour, county lines drug dealing. Lots of trouble in the town centre when the pubs and clubs kick out. It's such a shame because some of the surrounding villages are beautiful and Colchester itself has so much charm and history. But if you don't need to live in Colchester itself and can just use it for shopping and leisure (although even that is not what it was) then it shouldn't be too bad. Honestly though, it's not somewhere I'd feel comfortable having my teenaged boy growing up now.
Tiptree is fine, not particularly beautiful or exciting, quite a lot of post war housing stock, but nice countryside, Thurstable school was decent last time I looked, nice community feel, if a little cut off. You'll spend a lot of time driving teens to Colchester or Maldon as there isn't a great deal goes on in Tiptree. Not the easiest place to commute to London from.
Hatfield Peverel is a lovely civilised place. Almost all of it is very pleasant, there isn't really a 'rough' bit. It's walkable to the station from most places in HP itself. Anyway closer up the trainline to London is probably going to be out of budget too. No senior school so you'd end up in either Witham, Tiptree, Sandon or Chelmsford.
Witham is more mixed, but the nice parts are lovely and it has a great little high street for most of your needs. I'd rather bring up a teen in Witham than in Colchester.
Maldon is a great little town. Really pretty, lovely high street, nice community. Driving to the station will be a pain, but if you only have to do it four days a week, perhaps not too bad. It's more affordable because it's a bit cut off on its own, a bit like Tiptree. If Maldon seems a bit far look at Heybridge as it's a bit closer to Hatfield Peverel Station but still close to Maldon for amenities.
Coggeshall is really lovely, and Kelvedon is nice too, train station at Kelvedon. Honeywood school always seems highly sought after, but I am not sure there would be much in budget there.
Manningtree is lovely but that's a much longer commute. Although if you can be reasonably near to the station so saving money on parking and petrol, and get one of the fast trains from Norwich it may end up no longer or more expensive than having to drive from Tiptree to Witham or Maldon to HP, in the end.