The 450 class RC helicopter is a good size for starting out with FPV heli piloting. It's large enough to carry the FPV gear but not to big to be a threat to ones self or others. With a good set up it can fly for 12 plus minutes which is pretty good for a heli carrying the added weight. The T-Rex 450 or comparable clone is what I would recommend mainly because of the low cost, cheaper and easy to find parts. I personally like to use the V2 series with belt driven tail and aluminum frame. I like the aluminum frame versions for their durability even though carbon fiber is more popular.  Like the parts we make aluminum is strong and re-shape-able if it should be damaged by a crash where other materials may fail.

                          

                                                                      FPV 450 Heli videoing RC racing