1) I would go somewhere between the 2, 75-1 will give you a bit more protection but avoid oiling the plugs at slow speeds.
2)On a lake an anchor is not needed unless you want to you actually use one for anchoring. On the sea they are used in an emergency situation to stop you drifting. but on the lake you can row to the shore in a minute or so.
3) You can launch at a couple of places on Ullswater, but Parkfoot is a great place. For £5 you get lake access, parking, picnic area etc. You can then cruise the length of the lake down to Glenridding for an ice cream or a butty.
4) Your sib will do about 20mph flat out with a couple of people on board. If you have a smartphone try downloading a GPS app to give you a more accurate speed. But Ullswater is a quiet place, as long as you stay away from other lake users and act sensibly you can open the taps every now and again.
I clocked the ferry at 18kts earlier in the year !
I'm up there all weekend on Parkfoot, group of 4 families. Probably taking a couple of Sibs - Red Zodiac Futura and a Grey Wetline, give me a shout if you need any help.