Luden's Echo provides everything you need as a starting item, especially in this meta.
The meta is too burst-heavy for Rod of Ages, thus why even the champions that clinged to it (Ryze, Cass, etc) have dropped it.
Luden's provides 20% CDR, helps tremendously with mana, and extends your poke/burst damage with the passive. It is easily the best choice of items.
Most of the others need a buff to keep up.
Archangel's Staff is fine as an alternative, but you need a spammable spell. It can work with Karma somewhat but Luden's with her Q (especially Ult-Q) is really good.
Her shield is strong enough that you do not really need the Angel's staff and with her healing from Ult-Root, she should be fine.
GLP is underwhelming to me.
Certain mages can go Hextech Protobelt however since it is cheaper and does a lot if you want to close on them.
Karma might be able to use that but it feels like a waste with the movement speed and you would miss out on a lot of poke and some CDR with Luden's.
Her auto-attack effect goes really good with Kleptomancy. You can use Iceborn Gauntlet as a first item if you want to focus on that and are laning against an AD champion especially. This is more true for Karma top than mid but should be okay against some assassins, assuming you can live through the burst anyway. It tends to work better on melee champions you can poke a lot with autos though without risk of getting burst out.
Lich Bane might be okay as well as the full AP/Attack Speed build if you want to risk it.
The problem here is that you will be really squishy in a meta that is full of ridiculous levels of burst damage and you are likely to just get crushed under the burst damage instead of using Karma's real strengths.
In the right comp and against the right team, it does work though. I have tried it out but I do not recommend it except in limited circumstances.
Tank Karma tends to be more viable, or a full AP, or even AP with Ardent Censor (assuming you have a decent ADC). Luden's into Ardent, Redemption, etc is decent as well.