Think carefully about the distance - the Vendee (La Rochelle) is a long drag from Calais/Dunkerque. You should consider at least one if not two stopovers. If you want just a night halt look in your CC France book for Le-Bec Helluin near Brionne. Quiet municipal site with good clean facilities and mains. They also have locally made cider, and the site is about €11/night all in.
Allowing £500 for fuel suggests you are not using chargeable autoroutes, so expect about 10% better mpg than you get in the UK (the roads are much smoother) and average speed of about 35mph.
For monies, get a credit card that doesn't charge for overseas use - I use a Nationwide Visa but others (inc Santander) do zero charge cards as well. I use this for all fuel (and autoroute) charges - fuel pumps usually offer or automatically communicate in English, but even if not they are quite obvious to follow. For cash use your cash card to draw up to €300 at a go. There is a major issue in Europe with forged banknotes at €50 and above. If you get them from an ATM they will likely have been scanned before issue - you usually get €20's and €10's anyway.
As others have said, shopping at the French super/hypermarkets is a pleasure compared with over here. The big ones are Carrefour, Auchan, E. Leclerc, and Geant. Of the smaller ones you will find Super U and Intermarche, and there are Lidl and Aldi in abundance (and usually nicer than in the UK.) Our experience having just come back from four weeks in France is that Carrefour are getting a bit pricey and quality is dropping; Auchan is a good average for prices and quality; LeClerc is a little more expensive but not always the best; Intermarche is the best of the smaller markets. Shopping hours are 09:00-19:30 for most markets, but at the larger retail parks they are often open until 20:00 or even as late as 22:00 at some. Only the small supermarkets (Intermarche, Carrefour Market, SuperU, etc) open on Sundays theoretically for the sale of bread from 09:30 to 12:30. Not that almost all shops and small supermarkets close for two hours between 12:00 and 14:30 daily, and many are closed Monday mornings as well. Note that during the lunchtime closure period parking meters don't charge either - so pay for three hours at 11:00 and you don't have to move until 16:00!!
Finally note that fresh French bread doesn't keep although you can freeze it overnight.