I was kinda looking to assemble a droid similar in appearance to this:

http://starwars.wikia.com/wiki/Sith_war_droid_Mark_I

and was kinda looking for suggestion on building this bad boy. Not really sure where to start.

As Player Character or as crafted pet?

Crafted Pet for the players. essentially an droid companion

Special Modifications supplement.

Advanced Combat Chassis + Elimination Directives.

That is the most combat-oriented base for a droid that you can build strictly by RAW.

While it isn't the Destroyer it resembles, if resources become available, armor and shields could help; "war" implies it will see combat, so making it absorb the most punishment, preferably for you, could help. Might get spendy, but you might do it in steps, over time. Since the droid is an NPC, and has onboard power, you might get to treat its shields as always active, at least to the extent a destroyer's appears to be; no one would say that a Droideka's shield is as limited-duration as the ones in Edge/Age.

Basically, you can make a droid look like anything.

But in order for it to be able to actually do anything like it looks like it should be able to do, that takes a lot more work.

Otherwise, it’s basically just cosplay in the game, and you can flavor it up pretty much however you want.

Special Modifications supplement.

Advanced Combat Chassis + Elimination Directives.

That is the most combat-oriented base for a droid that you can build strictly by RAW.

Exactly. Add decent armor and 4 internalized blasters, maybe a few cybernetic upgrades and your war droid is done. Always remember, basically all equipment can be used by droids too and used as internalized components.