I'm not brilliant on MK itself as I lived in the surrounding towns and villages, but it has great amenities, loads of clubs for the kids, great shopping etc. Its purpose built so can feel a bit weird (some call it 'soulless') but this doesn't make it a bad place to live. There are all sorts of green spaces, loads of cycleways and footpaths too. Nice areas within your price range: Middleton, Furzton, Grange Park, Shenley, Loughton, Kingsmead. I believe the rule of thumb is that estates with straight roads and those immediately surrounding the centre should generally be avoided.
Leighton buzzard/Linslade is nice. Feels a lot more like a 'normal' town, and has plenty if shops, green spaces and a leisure centre IIRC.
From the areas you've mentioned, I'm assuming you'd like somewhere with a station? Bicester is over that way, nice area, and within an hour's commute by train. If you don't need to be somewhere with a station, I'd recommend Towcester, Buckingham, Newport Pagnell.
There are lots of affordable areas in your price range within that commute distance. You could even go into Cambridgeshire - St Neots, for example.