I would imagine it is 8 km from the north train station of Charmartin and about 13 from Atocha. I really dont know how close it is to the conference centre as there is a metro station there. Otherwise rush hour starts at 6 to 7 and then lasts until 8 or 9, but taxis add mostly for distance in Madrid and only partially for time. This is as traffic jams are the norm in Madrid. But not at 6 am heading out from the city!

There are litterally hundreds of hostels around Atocha and up towards Sol. They are comprable to 2 and 3 star hotels and many quite charming, but not my expertise. I think doubles run 35 to 50e. The NH hoteles and Sol Melia are easy for booking in advance though.

NH Parque Avenidas apears to be near metro Ventas and the same metro line heads out to the "611" about 15 minutes further.

You can not take luggage on Busses in Madrid any longer. They are only allowed on the Metro. So you want to get near a metro and anywhere north of Atocha and you will be fine.