Jokes apart, keeping these things away from the incoming phone line is going to be very difficult. My house isn't particularly big. More over its congested with electronic devices, all with high EM emissions! :(
I think MTNL state quite plainly that they do not guarantee the download speed - but this is somewhat a consequence of loading ADSL as an after-thought over a telephone network designed for voice.
Unfortunately there is absolutely no way of figuring out how much b/w u will get until you try it with your own setup (I have so much static noise in my phone-line as is - I am pretty sure my triband connection would suck!) Although they do have this 15-day trial thing were you can pay 800-odd bucks (+ 5K refundable deposit) to try it out before you make the actual investment. Best of Luck...
- farazs