The FAA require you to be able to place a load on a target three times from different directions, I believe. Anywhere else, if you can't put it down where they want it you simply get run off the job!
There is no "best" helicopter - it depends what you need to do. Obviously, payload comes into it.
On small helicopters, the 500 is good, as is the longranger for looking out of the door, and many people use lines up to 200 feet, although it can be a matter of preference - one poster here prefers 170', hence his nickname.
The AS 350 can be good, but your seat is further away from the fuselage edge so you have to stretch out more. Because of this, the minimum line you should use is 120 feet so you can at least see it occasionally (and then only with the hole in the floor - 150' is better without it).
Other machines such as the 212 require the pilot to be in the left hand seat for optimum slinging, and there will either be an STC in the flight manual for this, or you must carry a safety pilot in the right hand seat where there should officially be a pilot.
Major pitfall - when learning, as you descend with your head out of the door, you will end up very much to the left of the load unless you make a correction - when learning to land from a hover in the normal way you do this without thinking.
Maybe set the altimeter to zero on the ground before you lift so you know when you are getting to the end of the line and can expect it to get taut, and you might want to reset it if the destination is at a different elevation (e.g. elevation + line length), but after a very short time you get used to it and can dispense with it.
Your downwash needs as little obstruction as possible - bearpaws and ski baskets just get in the way.
Always have power in hand!
Otherwise, mostly as for short lines (though I'm sure there will be other comments) and it will take you about a week of hard practice to get proficient. It's the sort of thing you only need to be shown a few times and then it's all practice.
Phil