Alamo Car Rental - Rennes Train Station through Priceline

Can't answer about Priceline.
Alamo is inconsistent across locations. I have had appaling service in Marseille from them, but never tried them elsewhere, and their France-wide reviews seem OK. Usual warnings apply: check the car for damages upon pick-up, and drop off during hours to have someone check the car with you present.
Rennes station is under renovation and there is a lot of construction around, which might be a challenge, but signage in French stations is typically good (look for "location de voiture" or "loueurs de voitures") and locals, if approached with "bonjour, excusez-moi", will see your tourist look and be helpful. So will SNCF staff.

Exercise caution when using a third party, such as Priceline, because when a problem arises, who is there to help you? If you are a fluent French speaker then this is not as much of a concern. If you are not fluent and there is an accident, mechanical breakdown, or even if your car is missing at pickup, whom do you contact?

With a 24-hour toll free number and an English-speaking staff, Autoeurope not only has competitive pricing but unbeatable service which is why recommend them.

If Priceline is a done deal, it’s too bad. I would steer you to AutoEurope, too. So if you can’t change, be sure every tiny mark is noted on the contract at pick up, take pictures at pick up and at return, get their insurance if you want it to be hassle free. There are a lot of complaints about companies coming back weeks after the rentals, claiming some sort of ding and charging the credit card. See elliott.org for examples of problems in the US and Europe. I don’t bother with my Amex insurance anymore but use AutoEurope’s full insurance since the rental companies became more aggressive a few years ago.

The Rennes station is like a scene from M.C. Escher if the construction is still underway, but you’ll get to Alamo eventually.