Basically, where do you want to go with this deck? Why do you want to upgrade? What is your electrum budget?
If you have collected some cards, and wish to start upgrading, I would suggest upgrading some (super)novas, which are not for this death deck per se but will be useful in many future decks that you may build.
If you wish to spend electrum upgrading this deck, you will invest alot of electrums in death, tying you to future death decks for a long time.
Basically, when upgrading your first cards, look ahead, and upgrade versatile cards that you can use now, but more importantly will be of use in the future!