Do you want to win or do you want some fun games? Same goes for your opponents.
Because Decurion is quite annoying to play against, and people who just want a good time with casual list will be frustrated to play against your army.
So - the ark - DONT GLUE DAMAGED NECRONS IN! First you will have easier time painting it - second - you can fit the big effing gun in their place - poof! instant doomsday ark!
Catacomb command barge - dont glue handhold for the lord in it and you'll have ability to swap
CCB for annihilation barge.
The
CCB is tough unit with great mobility, durability(read
BRB about chariots) and need to choose its targets carefully(anything that can break 13 armor easily is not a good one!)
If you go decurion way - ya need immortals - if you like them to march on and get into that sweet rapid fire than gauss, if they sit in the back field guarding your objective or doomsday ark/annihilation barge than tesla is good since it doesnt need to be in 12".
Get deathmarks after your decurion - they're fun

and good against monstrous creatures, and counter deepstrike and killing opponents deepstrikers on his turn(red the codex, and be ready to declare this thing, also warn opponents about it so no bickering will ensue)
Sword and shield lychguads need to get there somehow - but they are nice as backfield guards too(if you await some drops into your deployment) - if you don`t go over 5 models... Go for triarch praetorians if you want to get into the fight(jump infantry is love! jump infantry is life!).
Tomb blades - as said before me, get 6 - they're cheap and great unit(not to assemble and paint tough - thats a pain) get 3 with gauss and 3 with particle beamers - your only source of small blast`s - or go full blast frenzy for clearing hordes(keeps people on their toes, spacing models fearing being smallblasted)

not gluing weapons/magnetizing is good too(dont take tesla on em, its a waste)
About the box with
hq - get the barge, start collecting is good - but before you want the stalker(big model) dont take it.
Dont expect great durability from necron vehicles - penetrating hits will be your nightmare - so jink or be in cover - opentopped and some ap2 - and you explode on 5+, ap1 and its 4+.
And think something about the bases beforehand - warriors with their stance are awkward fit - so do some basing to compensate or reposition the legs(you`ll have 2 spare warriors in any other warrior box excluding that battleforce you got, experiment bending their legs in hot water - or cut at the knee and reglue crouching stance so the silver tide wont look so bland)