I don't think it's mad at all, sounds very sensible to me. There are so many costs associated with buying/selling property (E.A. fees, solicitors, surveys, mortgage fees, stamp duty etc etc), you really want to avoid having to move again in the near future once you have moved, and schools are a big factor in many people's decisions. It is definitely worth doing your research into schools, catchment areas, childcare options before you buy.
I guess I would just be cautious about setting too much store by Ofsted reports, things can change a lot and a school which is outstanding today could be in requires improvement in 3-4 years time by the time your DS is starting school, or vice versa. Ofsted ratings aren't the be all and end all either, some really great schools never get better than 'good' just because they struggle to tick some particular Ofsted box.
How far away are the different areas you are looking at - would the cheap area put you completely beyond the reach of regular family support in general, or is the expensive area just more convenient? Bear in mind that your and your family circumstances can change a lot in 3-5 years, you may change job, you may get a new partner, family may move away, so no house is ever totally future proof. Also, have you thought through whether now is the best time to get on the property ladder, I know Brits in general are obsessed with property ownership and think of it as THE investment you absolutely have to make as soon as you are able, but its worth considering what the market is likely to do in the new few years, possible mortgage interest rate changes vs rental rates, and add that to the fact you might want to keep some flexibility with your young son, it wouldn't necessarily be a disaster financially to not purchase for a few years yet, providing you invest your money prudently in the meantime. But of course if you are really keen to have a place of your own for personal reasons then that is different...