Saffron Walden is beautiful but very expensive, for 500k not sure what you'd find. It's a small market town.
Bishops's Stortford is similar to above but just as pricey. Maybe even a bit more.
I live in that area for over 6 years in my 20's and struggled to make friends and have meaningful relationships, despite being a social person.
Moved to St Ives and absolutely love it here. I have met loads of people. Locals are all very friendly and I have a wide friendship circle.
I'm glad we're not in Huntingdon as it's not great in the town centre but I do go there for some of the shops, that's about it. And the train into London Kings Cross is good.
St Ives does seem to have an older population if you roam out on Market days - Mondays and Fridays. Lots of elderly then.
However if you come into town on a Saturday there are more younger couples and families. There's also a surprising amount of younger people out drinking and socialising in the pubs at the weekends. I can think of several places in the town centre that get quite busy weekend nights, even in January. Loads of charity shops too if that's your thing. There's a few cafes, pubs, restaurants etc by the river which is lovely all year round. There's a mini holiday vibe to St Ives in summer which we love, with the Jazz festival by the river, lots of day visitors, Booze in the Pews (a beer festival in the local church! It's super fun and always rammed).
Transport links into Cambridge are also really good. Perfect for a day out with the children. Free museums etc etc. Park & Ride is fairly cheap.
Hemingfords, and anywhere closer to Cambridge are super expensive. Not sure what you could find there for £500k. Lots of nice villages near St Ives/Huntingdon but they are very quiet and you'd be in your car a lot.
Since we've moved here we've noticed prices in St Ives have risen too so not sure what the market looks like now for a 3 bed detached in the area but might not be easy to find something you love.
You'll get more for your money in Huntingdon so I wouldn't discount it completely as depending on where you look it's definitely not all bad. Just the town centre is a little dire, like many places in the UK I suppose.
Worth looking around Suffolk/Norfolk area for sure. Bury St Edmunds, Newmarket and further towards the coast maybe ?
I head Grantham is nice but might be too quiet ?