I you are looking at the Nissan Sentra GXE then it is a very good choice. I own a '00 Sentra SE and after 14000 miles I can say that this may the best econocar in the market right now. There was a rattle in the driver's side door which was fixed under warranty, other than that no problems at all. The GXE you are looking at has 127 HP which still is more than that of the Civic DX (115 HP) and the Protege DX (103 HP) you are also considering. If I were you I would test drive all the cars and then choose the one you like best. If you can overcome the funky rear styling of the Sentra then you can't go wrong with it.
If you'd want to buy accessories for your Sentra online then may I suggest this website.
They have a wide selection of OEM accessories and for a limited period they also offer free shipping, by UPS ground, in the continental US.