Rengar is a great ganker for obvious reasons. The jump is a fantastic tool top and bot lane ganks, especially if the enemies decide to run though the bushes to try and lose you, which would work against anyone but rengar. I find that if your lanes are pushed it is not necessarily a disadvantage for ganking, just go up the lane into the side bushes, if the minions dont see you they will have no clue that they are about to get pounced on and bursted down. If you dont secure a couple (2-3) kills during the laning phase it can be difficult to keep up, but simply ultiing and picking off the fleeing squishies after a battle can roll you up into a ball and push you down a snowy hill. That said, rengar is a pretty situation champion in terms of kill opportunities, but the more fed you are the more possibilities there are, keep that in mind.

Because your jungling and not laneing the ad and as items are better for ganks, you have enough sustain and armor in the beginning for you to not have to worry about dieing in the jungle. The boots 5 are great for getting around the jungle, chasing, and the boots along with your ulti and get you an easy kill on a fleeing adc that should have been long gone. The bonetooth is very useful if your ganks are going well, but if they are not, dont be afraid to sell it. After you pick up your bloodthirster you will have sufficient enough damage to pick off squishies and you may want to start building armor, mr, and health from their, but if you want to keep decimating, more dmg is good.

Runes, in my opinion are just preference, with the bone tooth and boots 5 there is no need for any move speed tho. Id recommend ad and cdr runes just as a general direction to go.

One little extra note about is ferocity is, i like to get 4 stacks before i gank or initiate, b/c 1. if they dont see your big orange stacks to start they are less likley to run straight away 2. it allows you to toss you e (slow) before you burst them down your a double q, no 3.

gl, rengar is a ton of fun