For what it's worth, for the the last 3 years I've stayed at ibis Brussels Centre Ste Catherine

- Cheap (50-60 euros a night)
- Walking distance from the first stop for the 71 bus
- Lots of FOSDEM people stay there, meet people at breakfast
- Walking distance to central Brussels

RE: public transport: the first stop is pretty useful, as the bus gets full extremely quickly!

https://twitter.com/dberkholz/status/297652567511531522
https://twitter.com/amedee/status/693349393328705536

Google maps of the 71 route: https://goo.gl/maps/Xw6Esf7ZpYs

The rooms are pretty plain but I'm not looking for much, just a place to sleep.