So I am starting to flesh out an Eldar Army

I currently have like 40 stands of guardians, 20 old falcons to use as wave serpents, and a scorpian SHT.

I have inbound 8 Wraithlords, 2 Phoenix Bombers, 8 weapon platforms, and 1 cobra SHT.

Probably then going to add 1 warhost and 1 avatar box from GW direct.

After that what should I look to get?

I would suggest you get at least 2 of the warhost boxes if you don't have any aspect warriors. That will give you 4 stands of each type of aspect warrior on the sprue, enough wraithguard to reinforce one of your guardian hosts, a full-sized ranger troupe, and plenty of jetbikes (two full troupes if you base 2 per stand). You'll end up with too many guardians, but there are worse things that can happen to you. The avatar is also a good idea (and that nets you some of the cool new exarch models).

A storm serpent and/or a vampire can be fit into most army builds. You may as well try to get some firestorms to go with your falcons (maybe convert some to match the old style falcons that you have). Picking up 4 wave serpents couldn't hurt either, but not everyone uses them. The dire avengers/fire dragons blister is also a good idea, as both of those are good units.

After that, you'll need to choose your own adventure - 2 scorpions or 5 fire prisms, 2 revenants or 1 phantom, or maybe throw in a void spinner.

By the way, you'll need to pick up one more phoenix bomber to have a 'legal' squadron, if that matters to you.

I was thinking of just using the guardians in the warhost box painted blue with white helms for dire avengers and orange with yellow helms for fire dragons since those models seem pretty close and I have my very different looking old guardians.

Cobras are far (really far) better in squadron of 2.
And Phoenix flights counts 3 aircrafts, so you need a third one.

Looking at the rules I can see what you mean about the Cobras being more effective in the group of 2. I do not think that I noticed that even though it is a titan killer it has an AT of 6+ as a single shot but 2 gives you 2 templates with AT of 5+.

The phoenix blister I got was an old one so it only has 2 in it so I will have to find another one as you say.

you don't necessarily have to get 2 warhost box sets as Aspects can be mixed and matched, plus there's always variant lists which have smaller Aspect formation sizes (Ulthwe for instance).

IMO, the best Eldar units are:
Nightwings
Vampire raiders (for delivering aspect formations)
Firestorms
Falcons
Warpspiders
Rangers
Wraithguard
Storm serpents
and possibly Nightspinners

to start with, I'd avoid Cobras as they're hard to use well. Phoenix bombers are good, but hard to fit in a 3000pt list.
A good basis would be a couple of falcon fms with firestorms, couple of units of guardians and aspects (mechanised or in a vampire), a night spinner fm, nightwing fm, a unit or 2 of rangers then pad the rest out with whatever else you have. I like going troop heavy with a storm serpent to deliver amped up Guardian fms and a vampire with warpspiders for harassment raids. Rangers for the irritation value, guardians holding the blitz or moving up on foot and the falcons darting around shooting up transports and tanks (with the firestorms and nightwings providing amazing air cover). Then find a broken fm within range and shoot at it with your Disrupting Nightspinners. Doesn't always work (failed initiative rolls being my bane) but it's fairly reliable.