I think Darell is really good at differentiation. DD has had some really good opportunities while there - stuff like lessons from a secondary Maths teacher and other enrichment activities. They have a really wonderful music teacher. Every year learns a different instrument in KS2 so they get the opportunity to try several out. They do French in KS2.
I think all the state primaries offer some foreign language and I am not sure how much it is necessary, tbh. Secondary schools will assume no prior knowledge in any case.
Class sizes haven't been a problem for us. DD has been in classes ranging from 18 to 34 (very briefly) at different times and quite honestly, the smaller classes were worse for her. Children need to be able to spark ideas off each other and a big class means more potential friends.
I'm not particularly keen on the head at Darell, but the actual teaching is mostly absolutely brilliant.
Realistically speaking, you will probably only have the choice of one or two schools in any case from the state sector. Most schools' catchments are pretty small. And you know your children best - are they very bright? If so, you don't need a prep or tutoring. Are they likely to need 'pushing'? If so and you want a selective secondary, you might need to think about tutoring or opt for private (but that's obviously a lot more money).
I don't personally think a private primary is worth the money but many seem to. DD had a girl in her class who joined from Kew Green Prep in Y2 (the family has since moved out of London) and she was not ahead in any way, nor did her mother think it had been worth the money. The mother thought the teaching at Darell was much better!
And of course, I only have experience of one school - they are all different!