When a child is a toddler and you are looking, it is hard to think about what you want at 7 or 11 or 13, but you really must. Nth things you want for your 3 year old entering kindergarten are not the same as the things you'll want at 11 or 13.
So unless you are looking just at pre-prep which requires you to move on at 7, you need to think much longer term.
Personally I think that when starting so young, going for a Prep which feeds multiple schools is the best bet - you don't know how your child will turn out or which senior will suit - but ensure you pick a Prep that sends a decent amount of children to your aspirational choice of Senior, so that is at least an option. You really do have to think forwards first and then work backwards.
Fewer and fewer Preps go to 13 and by the time your little one is 13 it will be fewer still. Most children from Preps now move on at 11, but consider if you want a traditional boarding school which only starts at 13. This might influence your choice of Prep.
I'd also be open to the idea that you might move your child as their ability and personality becomes clearer. You might decide at 7 that a different Prep is more or less academic and more suited to your child.
Things to look at when you visit - the usual stuff about facilities and staff seeming friendly. I've also asked to see marked work and asked very specific questions about what prep is done for entrance to the next stage and no.s of children moving onto different schools at the next stage.
Personally, we went state until 7 and then into a 2-13 Prep. We knew which Senior school we wanted and although it had its own Prep, we didn't choose it, as we felt the 'tied' Prep had less incentive to genuinely prep children - those kids didn't have to do the entrance exam for the senior (a big advantage in the eyes of many - many choose 'tied' schools for an easier ride into the Senior). We picked our traditional 2-13 Prep for the full to 13 experience - it was a big Prep of 3 forms in each year, allowing for enough children for setting and sports teams and meant they had proper specialist teachers for science, languages, music etc from Yr 4 rather than the more classroom teacher based learning which is more evident in lots of Preps which go to 11. The existence of Yr7 and 8 in a Prep makes it very different.
The Preps Inwould avoid are the very small 'cosy' ones (often struggling) - they might appeal when your child is 3 but for a big 11 year old can be limited. I'd want at least 2 decent sized forms ((15+) per year group - otherwise genuine setting and getting enough sports teams out (especially if a mixed school) is difficult.
If really what you want is just a relaxed childhood and running around outside, I might wait until 7 to go independent when you might be able to choose with a bit more certainty about what suits you and your child.