I like to place one offensive ward and one defensive ward. The flask easily pay off for itself and with the flash you don't really need more than two health potions.
Alternative Starting items:
I personally do not use this. Maybe if you are new or something but with the flask you usually do not need 5 health potions.
This is mostly part of the old meta. You no longer need boots to start off but it is viable if you choose this path.
Choose your boots based on how you are going to play. If you want to gank alot the mobility boots are good. If you want the extra damage and know how to gank without the moblity boots, choose the sorcerers shoes.
Items that are good on evelynn:
This item is very nice, it gives flat mr pen, some health, some ap, and a passive that is amazing. DOes wonders on people who stack hp and since you are a low cd mage it's passive is preatty much permenant during the fight.
This works very well with liandry's torment because the slow means more damage from the passive. Also it gives you some more cc, and gives you a little bit of tankiness.
Good item on all ap champions. It gives you tons of damage. Simple.
With the changes to how flat and percent penetration works, this item has become very good.
Good defensive item. It is a good finishing item in order to let you play more aggressive knowing that you can really on your passive. Like if you can trade a kill for your ga, its worth it.
With the changes to how flat and percent penetration works, this item has actually gotten somewhat nerfed. It is still a very good item because it makes you tankier in lane and give ap. The flat penetration is okay but since it is an aura it has lost its value because now in calculation magic penetration it has become auro, then percent pen, then flat pen.
This is a good item if you want to burst down 1 target, like the ad carry. Gives alot of ap and is a good item overall.
This item is good. It gives ap, some armor, and a nice active. If you don;t need the active I would recommend building the other items mentioned.
This is a viable item but I don't like it too much on evelynn. Its nice for long fight where you can heal yourself but evelynn is mainly going to go for burst fights so it depends on the game. Situational item.
Gives hp and makes you tankier. Like a replacement for ga. Its a good defnesive item.
This is a good defensive item. Gives a good amount of armor, good hp, and a nice active considering you are a melee ranged champion. Very good item and very recommending if the opposing team is running an ad mid.
This isn't a very good item on evelynn for the price but if you ever hit a 90 minute game and you feel like it go for it. Otherwise I would say not to use it.
Ideal final build for average game:
This build is ideal because it provides most of what you want and it works together. With about 40 flat magic pen and 43 % magic pen, mr becomes a thing of the past for you. You are somewhat tanky to make up for your natural squishiness and you can deal loads of damage. This build is for the average game, change it wherever necessary.