Congratulations! It will be a fascinating trip for you.

~ Is early Mac a good time to visit Languedoc/Provence?

Yes, early May will be perfect. Expect bright sunny days, warm but not hot. At the Cote d'Azur, east of Marseille, seawater will be warm enough for swimming.

~ Do we need two plan for separate base camps (one for Languedoc and another for Provence), Any recommnedations which of the cities will make the most sense as a base?

The area which you will cover within 6 days (full six days or including travelling days?) is huge, so I strongly advise to restrict yourself a bit. Here my personal ranking:

First-rank destinations:
Carcassonne, Arles, Nimes - and I would include Avignon

Second-rank destinations:
Montpellier, Aix au Provence, Marseille

Third-rank destinations:
Beziers, Canal Du Midi (which is hardly a destination in its own right - unless you rent a boat)

And I would strongly suggest to rent a car. Theoretically, you can reach the cities by train, but in order to explore quaint villages, archeological sites and the scenery, you need a car.

I thought a bit about your itinerary. What I would probably do:

For the first four days I would stay somewhere in the triangle Nimes - Avignon - Arles. If you rent a car, it is a matter of preference whether to stay in a larger or smaller town. Many Fodorites opt for St. Remy (which is strategically located in the center of the Roman triangle).

After thinking a bit, I would recommend to skip Carcassonne - to much driving. You can visit Aigues-Mortes instead - it is a good substitute.

For the remaining two days, Aix-en-Provence or a village in the Luberon would make a good base, e.g. Gordes.

However, if you want to see Nice and Monaco, Marseille is too far away. You need at least three hours from there to Nice.

I would probably skip the Luberon and spend the remaining two days directly on the Cote d'Azur. If you prefer this option, I can give you further help.

There is no need for a private tour service. You can use the internet to book all arrangement. You will survive with English.

A good web resource is www.viamichelin.com. It provides map, guidebook, hotel guide and restaurant guide.

BTW, open viamichelin, enter a destination, e.g. Arles, click on the map and zoom out. Then you get an orientation of the area. viamichelin is a route planner, too.

~ What is the most cost effective way to reach Paris/Frankfurt from Marseille - by rail or by budget airlines?

Budget airlines. If you book early, even Lufthansa often has good deals (also try return flights - they are often cheaper than one-way-tickets). Think about skipping Marseille and flying out of Nice.

I agree with the Arles, Nimes, Avignon triangle. In each of these are buildings going back 2000 years and yet each is small enough to walk around in a day. Parking is small problem as Avignon is a walled city and you tend to leave cars outside and nothing is well laid out for cars so you could dump a car and use the trains for this part of your break. This would restrict your ability to visit Pont Du Gare which is a Roman water bridge (carries water over a valley) but this can be done at the end/start of the visit or by a local mini-bus.

You might also want to visit the Camargue, famous for sea scapes and wild horses.

If i wanted to do carcasone i might also do beziers where there is a steep series of locks which as a walker might be of interest.

Toulouse airport, oe Carcasonne may be the way to Paris

I agree that Avignon and St Remy are not to be missed.
I did all of the above, practically, on a River Cruise with AMA WATERWAYS. Transferred from Barcelona by Carcassonne, boarded at Arles and sailed up (7nts) to Lyon. The train to Paris.
Truely an AMAZING trip. In June.... Lavender!!!!! And the WINES!
Enjoy!!!

Thank you TVLIRISH, BILBOBURGLER and most of all Ecnaton for the detail information. I am checking outsome of the information as it comes.

Some further clarifications: My 6 full days in these areas include travelling from Barcelona to the Base Camp and from Base camp to either Paris or Frankfurt.

Will it make more sense to travel by train to the base camp and then rent the car for the daily excursions around base camp?

Regards

>>>Will it make more sense to travel by train to the base camp and then rent the car for the daily excursions around base camp? <<<

The fastest option would be renting a car in Barcelona and driving. Driving time to, say, Avignon is about 4 hours. However, you have to pay a dropp-off fee for cross-border rental.

Another option is taking the train from Barcelona into France and rent your car in France. Be aware that the train takes 7 to 8 hours from Barcelona to Avignon.

You can save an hour or so, if you take the train to Perpignan and rent a car there (there are rental car stations close to the train station). Unfortunately, there is no rental car agency in Cerbere, where you have to change trains.

Another option would be taking a Vueling flight from Barcelona to Nice. In this case, the following itinerary could work:

Days 1-3 in Nice, on the third day driving to the Provence.
Days 3-6 in Provence.
On day 6, you can take the train to Paris. The train ride from Avignon to Paris Charles de Gaulle Airport is just 3 hours.

I would definitely go to Carcassonne, but I'm biased since I live here. The train from Barcelona to Carcassonne isn't nearly as long/grueling as the ride to Avignon. You can spend the night in the medieval castle or at a vineyard like mine, taste some good wine, visit the castle ramparts, see the canal du midi, and then be off on a short train trip to your next destination like Avignon or Arles.

You could also drive from Barcelona to Carcassonne, which would afford you a shot stop in the Roussillon which is pretty gorgeous. Although you might be Catalan'ed out if you already spent time in Barcelona.

More info about wine-related stuff in the Languedoc at my website: http://love-that-languedoc.com

We took the train from Estacio Franca in Barcelona to Perpignan where we picked up a car. Had lunch in Perpignan, drove to Arles and spent a couple of hours, then went on to Orange. A longish day but not unbearable, and Avignon or Nimes would be less distance. We saved more than US 200 for a five-day car rental that ended in Paris, compared to renting in Barcelona. And that was after the cost of two first-class rail tickets. The train we had, leaving at about 8:30 a.m., was neither fast nor luxurious but it was spacious and comfortable.

That time of year you'd be best to reserve a car ahead - most of the majors have offices at Perpignan. You still may not get exactly the car you wanted, but at least you'll be first in line.

We bought a Europe chip for our GPS and brought it along. It worked very well and made us much more adventurous. We brought it because the fee for a one way rental was pretty high at the time with Hertz.