Some good info already provided and only a few snippets to add.
1) – March is my own favourite moth for trekking, mornings tend to have blue skies and are often quite warm, cloud tends to build around the peaks about mid-day and drops into the valleys as the afternoon goes on with a chance of rain (snow at altitude) in the late afternoon / evening – Not as many trekkers about, so no problems finding a bed for the night and no huge queues for meals.
I have also trekked a few times in February which is colder, but as long as you are well prepared with good kit then it isn’t a bad time to trek in Nepal
2) TIMS and ACAP – TIMS is $20 if you don’t take trekking staff and $10 if you do, ACAP is 2000NPR (Around $25)They can be purchased at NTB in Kathmandu or Pokhara
3) You can camp at one of the lodges at ABC but there is little point as you have to pay a fee for your tent, so as accommodation in the lodges is cheap it isn’t worth all the hassles of lugging a tent and all the associated equipment with you
4) The Kanti Path – 06.30 reporting time, 0700 departures
Good Luck and Happy Safe Travels
My Nepal Blog members.virtualtourist.com/m/50192/6ec/