Sounds like a brilliant idea - For a first longish tour - down the coast to Biarritz is ideal – If going from Calais I would follow the coast south and west so that you can drop down onto the Loire to Nantes and down the coast –personally I would take the ferry from Portsmouth to Caen or St Malo and go straight for the coast north of Lorient. Fitness wise if you are young I would not worry about it too much – you will get fitter and faster as you go - Forget about trains unless the weather gets really rubbish - top of France to Biarritz along the coast will be dead easy over two to three weeks - can be done without too much effort from Brittany in one week - need to get basics -
bike does not need to be that good – just well maintained- (on my first tour at 15 years old my mate that came with me did it on a currys three speed – 50 miles a day across germany) – fairly standard second hand mountain bike with rear rack and panniers will do – Halfords panniers with some good plastic bags (Tescos rubble sacks) -share out the tent(s) between you and shove on top of rack - if its in your budget put some tough semi slicks on - go over bike before departing to make sure that everything works properly
tent - cheap dome tent from tescos, millets etc - weather along coast in October will generally be relatively warm interspersed with low pressure giving occasional periods of sustained rain - brand new cheap tent will cope if you are careful about where you pitch it - wicked view over the sea often turns into mega unsheltered position if wind gets up
Sleeping bag - again something cheap and cheerful -Tescos?- its not going to be very cold at that time of year – make sure its packed in a good plastic bag – cheap roll mat of the horrible bulky sort will be adequate – strap it on the back over the rack in a bin liner
Cooking – if you are on a budget cooking for your selves is the way to go - sort out what you like and are capable of cooking between yourselves before you go – and work out what you will need to do the deed – the weight can be spread between you – a trangia will do the job for a stove – myself I prefer a low gas stove – not expensive in the context of the whole – if gas then camping gaz is most easily found in france – pasta is an easy meal to cook and provides all the carbs you need – but after a week it starts to become a bit boring – shops in france have plenty of variety with fresh food
Wear whatever you think will be comfortable for cycling – but in my experience what stops me at the end of day is not that my legs are tired but the fatigue in the contact points – saddle, hands and feet – consider padded shorts – good solid footwear and cycling gloves. It is unlikely in October that you avoid rain so a waterproof coat, leggings and a hat are a good idea.
Finally in October you will find a lot of the coast is not actually shut but sometimes appears that way – good idea to research campsites if you are using them to find the ones that are open. As has been said before target the municipal sites – facilities are more than adequate and cheaper
Whatever the final outcome be it that you do it all the way by bike or a combination of trains and bike – enjoy it
(as an aside what course are you doing that has a field course in Biarritz – a cold two weeks at Malham Tarn in the snow at Easter was mine)
bike does not need to be that good – just well maintained- (on my first tour at 15 years old my mate that came with me did it on a currys three speed – 50 miles a day across germany) – fairly standard second hand mountain bike with rear rack and panniers will do – Halfords panniers with some good plastic bags (Tescos rubble sacks) -share out the tent(s) between you and shove on top of rack - if its in your budget put some tough semi slicks on - go over bike before departing to make sure that everything works properly
tent - cheap dome tent from tescos, millets etc - weather along coast in October will generally be relatively warm interspersed with low pressure giving occasional periods of sustained rain - brand new cheap tent will cope if you are careful about where you pitch it - wicked view over the sea often turns into mega unsheltered position if wind gets up
Sleeping bag - again something cheap and cheerful -Tescos?- its not going to be very cold at that time of year – make sure its packed in a good plastic bag – cheap roll mat of the horrible bulky sort will be adequate – strap it on the back over the rack in a bin liner
Cooking – if you are on a budget cooking for your selves is the way to go - sort out what you like and are capable of cooking between yourselves before you go – and work out what you will need to do the deed – the weight can be spread between you – a trangia will do the job for a stove – myself I prefer a low gas stove – not expensive in the context of the whole – if gas then camping gaz is most easily found in france – pasta is an easy meal to cook and provides all the carbs you need – but after a week it starts to become a bit boring – shops in france have plenty of variety with fresh food
Wear whatever you think will be comfortable for cycling – but in my experience what stops me at the end of day is not that my legs are tired but the fatigue in the contact points – saddle, hands and feet – consider padded shorts – good solid footwear and cycling gloves. It is unlikely in October that you avoid rain so a waterproof coat, leggings and a hat are a good idea.
Finally in October you will find a lot of the coast is not actually shut but sometimes appears that way – good idea to research campsites if you are using them to find the ones that are open. As has been said before target the municipal sites – facilities are more than adequate and cheaper
Whatever the final outcome be it that you do it all the way by bike or a combination of trains and bike – enjoy it
(as an aside what course are you doing that has a field course in Biarritz – a cold two weeks at Malham Tarn in the snow at Easter was mine)
!! 
