Dont buy the Lot. It's not worth it. The price aint that good and multiple footslogging tyrants are also not optimal.
You dont have the gaunts to feed the tervigons. For 1 Tervigon you need at least 40 termagaunts for it to be worth it. For 2 Tervigons i'd recommend at least 60, more like 90 termagaunts. If you're not willing to field many many basic termagaunts you do not want to field Tervigons. If there are no units of termagaunts to replenish they're completely worthless.
Since your Army ist so small you have multiple options from here. In my opinion you cannot have enough Carnifexes. Consider buying another 2 boxes If you want to field old one eye.
Build them with dual scything talons, bone mace, tusks and spore cysts, maybe one or two with scything talons + crushing Claws. Old one eye needs melee fexes around him to Work.
Genestealers are about the best melee unit in Game. They are brutal and some more could be beneficial.
The good old termagaunt bomb still works but only in Turn 2 now. 30 gaunts with devourers delivered via trygon put out a scary amount of dakka right where you need it and without taking casualties beforehand.
Some venomthropes are also something to consider. -1 to hit Aura is super useful.
Personally i'd just buy as many Carnifexes as possible but then again i'm a Big Bug Player and might be biased