Firstly I'm not in either Berks or Bucks, so this is more in the way of general thoughts. I didn't worry about secondary when my daughter started primary, mainly because we had a bad enough job finding a place at primary and then moved our daughter part way through Reception so secondary school whilst at the back of our mind wasn't our immediate concern.
I daresay quite a lot of parent do worry though so you shouldn't feel too bad about this. What I would say is your child settling into primary should be your main concern at the moment. This is a huge step particularly for the eldest who have to go it alone. Younger siblings get a slightly easier job of going to school, after all they've probably done the school run for a few years first and are familiar with the environment.
Unless the secondary schools in your area are all on the Ofsted list of failing schools or your daughter is already showing signs of being a budding Genius (no I'm not laughing), such that she needs a particular environment then it surely makes sense to see how she settles and makes progress in her primary before you take the plunge and uproot the whole family.
Presuming that you are happy with your chosen primary school you should then check out the secondary schools in your area first. Also find out where children from your primary school invariably go to secondary, if you have a mixture of good and bad schools in your area, then find out what percentage go to each, this will give you an idea of the standard of your primary.
Finally, when your daughter starts primary, don't be fobbed off by the stock phrase "she's doing very well", make sure you know where your daughter is in relationship to her peers.
If you ever feel worried about your daughter's progress come back to this forum. Everyone here had already done Primary so I'm sure advice will be more than forthcoming.
Finally, with Messrs Blair and co. currently messing BIG TIME with the Education system I would wait and see how this is going to have an impact before you do anything drastic like moving.
Hope this helps