I think £300k would get you a three bed house with garden within a mile of the station in Harpenden. But it would be a terraced house. An end terrace on the street backing on to mine was up for £320k recently.
There's a real price hike going from terraced to semi or three to four bedrooms, or modern to period. If you don't mind having to drive to the station you get more for your money, unless you choose the Roundwood area where school proximity outweighs station distance. And yes, getting on the train at Harpenden gives you a much better chance of getting a seat. Also a good chance on the way back since you'll be getting on at Faringdon. But the journey's so fast it's not the end of the world if you don't get a seat.
Only part of Harpenden to avoid is Batford - on the eastern side. And it's not rough, or anything, just less desirable.
Basically even the worst-performing primary school in Harpenden (in Batford) is better than national average. There are three state comprehensives, all with good results (Roundwood and St George's are best, Sir John Lawes not as sought-after but again better than national average and good for less academic kids, I'm told).
KTeePee is right about restricted school choice if you move to the villages.
It's a small town full of relatively well-off people who want to send their kids to good state schools - as an indicator local supermarkets are Waitrose, M&S and Sainsbury's. Every month there's some entertaining small town event going on, like the Highland Gathering (seriously) or classic car rallies. There's a regular farmer's market. And more children/parent activities than you can shake a stick at.
You could buy our house, if you like, I'm bored and want a new one...