This is a work in progress I will take comment and suggestions try em and incorporate them into the build. Much more to come

Varus is a melting pot of sorts as far as hit abilities and style of play. His Q Piercing Arrow screams Cait but his personal twist is that his increases range at a the cost of movement speed. His W Blighted Quiver (Passive) creates a twitch like mechanic damaging the enemy that provides a detonation mechanic, landing any spell on a blighted target triggers it (This is similar to Twitch's Passive + E). His E Hail of Arrows resembles Miss Fortune's E (Make It Rain). The only thing unique about Varus compared to most AD Carrys is that his ult (Chain of Corruption) features some hard AOE CC (closes thing in a carry would be ashes' ult). This is a rare treat and allows for some strong lane control at level 6+ making the lane very easily gankable even if it is not very pushed. So in summary you can draw the skill set required to play Varus from you experience playing other carries who's moves got plagiarized.

Disclaimer: You should never be rigid in your choice of items, based on what your team has and what the other team is building you should adapt to any situation.

Doran's or boots/pots first - personal preference in term of how aggressive or safe you want to be also is creatly effected by who your support is. IE if you have Janna, Doran's Blade Since you know you are slightly quicker. Grab the one you didn't grab next. If harass makes you go back and you don't have enough for something big grab another Doran's and help your support with more pots or wards.

IE,PD,BT - Infinity Edge, Phantom Dancer and Bloodthirster should be your next buys IMO They order is personal preference except I do suggest getting at least a BF first to pack more punch. I won't go into to much detail here if you don't know what these are you might way to spend more time playing some of the cheap cd carries like ashe instead of dropping 6k ip on Varus, or spending hard earned cash. IE is always a heavy hitter and PD is always a must run speed crit and did i mention you are on speed (stacked with you passive it gets pretty sick). Lets be Honest BT is like a perma-health-pot.

After this again personal choice. I've been playing very tanky teams recently so I've been a fan of picking up Madred's Bloodrazor. Varus's passive attack speed coupled with PD and Madred's makes him a great candidate for this item. You can even do a little AP mix like a teemo if the situation dictated.

Lastly if the game lasts long enough to get a full build or your just insanely fed grab a Guardian Angel. Your a carry you get focused. Play it safe.

Other Great Alternatives - Black Cleaver, Trinity Force, Frozen Mallet, Maw of Maltorius, Last Whisper.