For accommodation, you have three possibilities:
1. Book a hotel in Paris, close to any of the RER B stops, and then come to campus by RER B followed by bus or stairs.
2. Book a hotel in Massy, close to the TGV train station, and then come to the campus directly by bus or by RER B + stairs.
3. Book a hotel in Orsay or Palaiseau and then reach the lab by RER B + stairs.
The second option is the most comfortable (but not necessary the cheapest).
You can find here a list of hotel close to the École Polytechnique.
Below is a list of hotels close to the Massy TGV station:
http://www.booking.com/hotel/fr/kyriad-massy.fr.html
http://www.booking.com/hotel/fr/sejours-amp-affaires-massy-atlantis.fr.html
http://www.booking.com/hotel/fr/adagio-access-paris-massy-gare.fr.html
http://www.booking.com/hotel/fr/residence-paris-massy.fr.html
