Yes, you'd be better off with Timid/Modest nature for Lanturn...And as for Hidden Power, if you would ever want to use one, use either HP Fire 70, or Hidden Power Grass 70. Ferrothorn walls Lanturn, and a stiff fire attack before a Rain Dance will likely OHKO it...
OR, if that nasty Ground/Water type comes out to troll you, just take it out with HP Grass...It's a simple solution...
252 Special Attack, 252 Speed.
Another option is to have another pokemon use Rain Dance and switch into Lanturn. A preferable option for something like that is a Gyarados or something, or possibly even that Politoed's Drizzle Ability.
However, using a Kingdra on the same team with a Rain Dance + switch in with Lanturn will really give you an edge...