where to stay in provence

St. Remy would be a fine place to stay. You can get off the train in Avignon and pick up your car there. If you're taking the TGV from Paris, even better since you can get off at the Avignon TGV stop, which is outside the city, and pick up your car w/o having to deal with city traffic. You can then drop off the car at whatever train station you'll be getting the train to Barcelona.

For the Cote du Rhone, I would consider basing yourself in Vaison la Romaine area and doing day trips out from there. For the area south of and west of Avignon, I would look at either Uzes, St. Remy, or Les Baux as a base to see this region of Provence. If you are trying to avoid basing in the big cities, you can't go wrong with any of these options.

:...to tour wineries....:

Rick's book on Provence includes a suggested loop tour of Cote d'Rhone wineries and villages. We used it as a rough outline and had an enjoyable day. (His recommendation for a visit to Domaine de Coyeux was excellent; the drive up to the winery is spectacular, and we enjoyed the tasting and even bought bottle or two.) The village of Gigondas was charming; wine-tasting opportunities, and an excellent lunch at a cafe on the square called "Nez!"

Pick a single base location based on the places you actually want to visit. Plot these places on a map and look near the center of the area that they describe.

If that includes a lot of the Rhone Valley sights, such as Arles and the Pont du Gard and Les Baux, then you might prefer to be a bit south of the Cotes du Rhone area in a place like St Remy, but everything is really close enough for either. Wherever you go, you will want a car.

I do not completely understand your automotive concerns. Returning a car to a different location than pick-up is not a problem in France, but in any case your train almost certainly passes through Avignon, where you can also pick up. Given your time and aspirations I would not try to connect these dots relying only on trains and buses.