Actually they have revised the plans so the above mentioned plan costs ₹1604
Now to answer your questions
The 50 Mbps speed will be for online streaming for ex YouTube etc for which Excitel has peering agreements
You can get VERY high speed while downloading from Utorrent etc as they also use peers in order to download data( in my experience torrent with lots of seeds can go as high as 11 MBPS(yeah you heard it right 11
mega bytes per second )
Otherwise you'll get 30 Mbps ie 3.75 MBPS on regular downloads such as I'm
2). Download speeds remain constant at all times the torrent downloading could go down a bit but you wouldn't even feel it. Rest remains the same.
3).There is no limit whatsoever believe me I have downloaded upto 1.5 TB in a month


4).As I said the plan costs ₹1395+taxes which amounts to ₹1604 , it is the most affordable price for such high speeds without fup that I have heard of .
Excitel provides seamless service and I hardly have an issue
PS. I guess you know 50 Mbps(mega bits per second) amounts to 6.25 MBPS(mega bytes per second)
1 byte = 8 bits.
Remember there is no extra payment for cable installation and if they demand some extra amount for setting up
router don't give them as setting up is very easy(log in to your router the IP address is printed on the back of router then login using id and password (also printed on the router ) select connection type ppoe and then type in the user ID and password (provided by ISP).
Voila you're all setup
I hope I have cleared some of your doubts