The cheapest champions are old ones that exemplify major roles pretty well and are pretty easy to play relative to their role. Master Yi has a good deal of passive strength and 3 of his abilities don't need a target; this makes him extremely easy to play if you're still getting used to the control scheme.
For nidalee, AD is good to get a high DPS and farm in the mid-game. Your other options are mana/CD for support/farm and AP for burst/range damage. You can't match the sustained DPS of AD nid until you get both CD and AP, which takes 40 minutes to get to good levels. People tend to prefer AP nidalee so that they can deal a lot of damage to champions from a safe range, because she's pretty squishy and doesn't have any good escaping until 6, plus her real AD power comes at 6. Personally I go mana/CD with a lane partner and spam traps and heals. AD nid is very rare because not many players both really want to deal damage to champions and are patient enough to wait until level 6 to do it at all. Most people are only willing to wait that long for a real assassin, like Akali, Katarina, or Kassadin.
