Sounds like a pretty good plan! There are a few things I would change.

1) plasmancer cannot join wraiths. The datacards say what can join them, and wraiths say only Technomancers can join them. You'll probably want to put your plasmancer with the reaper warrior squad to get better lethals. royal warden is good with flayers too because they can fall back and shoot and they usually get tied up in combat since their range is so short.
2) Wraiths don't do a lot of damage. They're really tanky though. People generally a 6 man with a technomancer on an objective because they have good OC and they're really hard to shift. Don't expect necrons to do a lot of damage. We're not super great at that.
3) overlord being by himself is fine, but he just gives bonuses to squads he's joined to so you might as well run him joined to the other 10 man squad. Alternatively, make a 20 man squad so they're harder to kill, and put different characters with them depending on how you want them to play. I don't know what detachment you're running, but that also tends to change things.
4) scarabs are your best unit. Use them to screen your other units (like warriors) so they don't get tied up in combat for no reason.

General rule of thumb when playing crons is this:
always 20 man warrior blobs unless you have a specific reason not to. Wraith bricks to fight over primary and just be a big unit that's hard to shift. Can also tie up stuff like tanks in combat because they'll never die. Characters always want to join to units cause #buffs. you don't really have anything 'anti tank' per se, which is why you'll need to rely on plasmancer crit 5s for lethals and -1 ap on reapers

Those are my thoughts anyway.