I play Rugby Second Row or Blindside Flanker, so I'll happy to help out as much as I can
I'm 6'3 and currently 104kg, which is on the lighter side for a second row, but I am more muscle than fat, so I smash the fatter guys lol
I definitely agree that if you want to play, then you might want to add some size, consider it padding for the hits you are going to give and receive.
Shoulders, Traps (Helps protect your neck), Pecs (Hits to the chest, tackling), plus Arms (Helps prevent elbow and bicep tendon injuries), Core and probably most importantly Back!
Rugby is a violent Full contact sport, so there is no point taking it lightly, you can and probably will get injured, so the best thing to do is injury proof yourself as much as possible.
Rugby is about Explosive Power and Strength as well as having good cardio vascular fitness, so your training will vary in the offseason a lot from in season.
Since you aren't playing yet, I would concentrate on Strength Training so the Program that Chewy has posted is probably a good start.
Remember whilst you are lifting EXPLOSIVE POWER
I would probably throw in some Hill sprints as well, great for Cardio and great for your legs...
If you want to know what a game feels like on your body, try this
Start at 1 side of the gym
Sprint to the other side of the room
Do 5 Clean and Jerks
Walk back to the start
When you get back to the start, do it again straight away.
Keep doing this for the length of a game....
(For added realism, get someone to punch you in the face every now and then)
Rugby is the game played in Heaven, Have fun and get big!