Get a GREEN one - red is no good, you won't see the beam
1mw is enough - that's what I use - about £30 to £60 depending on quality
Less on e-bay, but you take a risk it won't be good.
This is mine
2nd item down - got it for £30 off their stall at an astro event
Finders & Pointers - low cost telescopes, eyepieces and astronomical equipment from Astronomica
Sends a strong beam up to the sky, you can point straight ON to stars etc...
Very strong beam - in mist it looks like a huge light-sabre
Bear in mind, they don't like the cold - keep in a pocket, or it won't fire up until you warm it a bit (under the armpit for a coupla mins works wonders)