Hello! I am going to be starting a Descent into Avernus campaign soon, and I'm going to be playing a human wizard going the evoker route. We did session zero last weekend, and stats were rolled. I'm very happy with my results, but I'm having a hard time allocating the numbers! I took Magic initiate and tough as my starting feats to get some defensive spells and the extra hit points to mitigate the squish factor. I rolled, an 18, 15, 13, 13, 12, 11 for stats. My initial thought, with bonuses, was 11str, 15dex, 14con, 20int, 13wis, 12cha. A wizard's important stat is int right? So straight forward, until I thought of trying 11str, 18dex, 14con, 16int, 14wis, 12cha by changing the bonuses and moving around dex and int. Do I go for the pure blaster build with the higher int and risk dying more easily, or do I take less consistent spell damage for a higher AC and initiative? Help please! 

Having the top stat be int, is good.  Str & CHA can be your dump stats. I like your 1st thought/choice.  So you are good with that one if you do nothing else.

I assume you picked the sage background?  CON is good for both HP and concentration.  You could have with sage do a +2 to con, then your choice for wis/int.  So put in a 18 under INT, and boost it at the level 4 ASI.  Boosting CON might be difficult under ASI, and won't be boosted until level 8 or 12.  So give that a 15, and a +2 so you have a 17,

Plan to be in the back and just suck it up with a mediocre dex.  The other thing is to find a background that does both dex and con boost?

Having an 18 does give you freedom to ignore a boost to that stat.

As a wizard int gives you so much more than dex (save dc,hit bonus etc) and a dead enemy can’t deal damage

Keep 15 dex and take Mage Armour spell, that's still AC 15 which is fine. You can always increase Dex at 4th level if you want. There's also spells that can give more defense like Shield, Blur and Mirror Image. So, the 15 Dex is fine.

However, you're fine to keep Int at 18 until 8th level. While maxing your Int is good, the game is balanced around a 16 stat in early levels, so having 18 outta the gate means you're already ahead of the game. Use that +2 elsewhere or for Dex if you want. 

You can also ask your DM for a custom background (there's rules for it in DMG) to get the +2/+1 or +1/+1/+1 starting stat bonus that works best for you. Or see if your DM will let you ignore the ASI restrictions for your chosen background - if they do, add the background but ignore the ASI and leave blank, and on your sheet directly just click the ability, add the bonus there with a note.