Starting Items
If the lane will be tough - get more tangoes.
Early
~7-10 min
Treads switch and raw tankiness may be halpful for Spectre in situations when you desperately need HP
Core
~18-24 min
Radiance is the best farming item you can get, but it slows you down quite a bit. It`s better to get Soul Ring and Blade Mail most of the time (it will be enough for farming), but if you have a free early game and enough time to get Radiance then go for it.
Before Manta Style
ALTERNATIVES: Blade Mail, Philactery
~25-32 min
Increases our farming speed by a lot, gives us a lot of damage against solo targets, increases tankiness by a little bit
Gives a lot of stats and slow passive (which should work on illusions too), so it`s one of the best items to pick up actually
Situational Early
~5 min
Before Blade Mail
ALTERNATIVES: Falcon Blade
~5 min
Before Blade Mail
ALTERNATIVES: Urn of Shadows
It works with Dagger and Step, it's generally nice for stats and farming. But it also practically takes the same niche as Radiance/BM, so it depends
Alternatives: Radiance, Blade Mail
~8 min
It`s both farming and fighting item. Works well with Radiance, but is very timing dependent
Alternatives: Radiance, Philactery
Lategame
Gives a lot of tankiness and good for locking people down. Most of the time you don`t really need it or just have more important slot to have. Altho, it`s actually pretty good as 6-7th slot.
A lot of attack speed, damage and miss chance. May be replaced by Bloodthorn if you have enough money.
Situational
ALTERNATIVE: Skull Basher
Almost all the time you WILL need to pick up BKB. It helps a lot against disables, break and a lot of other stuff.
Heart is that point of becoming out-of-control. It replaces any of your cheap regen items, while also making you very hard to kill in fights (not only because of HP pool, but also because of ability to get out, regen ad re-engage).
Good armor aura and attack speed buff. Good thing to pick up if you don`t have it in your team and really need armor
Other Significant Items
~10 min
ALTERNATIVES: Meteor Hammer, Blade Mail
It has a fairly low cooldown and works well as regen item replacement and a farming item. You don`t really need to use it in fights, but you CAN if you need to focus a squishy target. It`s better to purchase with some cheap armor (like Blade Mail)
Gives a lot of tankiness and sustain, BUT doesn`t boost farming speed, while also being quite expensive.
Good for tower dives (and very agressive plays) or against summons/high attack speed.