So, I was trying the game again after failing a lot of times, this time with help of a good guide, but I had a doubt.

The guide in question had a short build reccomendation, but it was for the version with the DLC, which I lack, as result I wanted to know, what are the alternatives to the xenarite upgrades for the builds in question?

Like, the ones with xenarites in the guide were

Healer: Xenarite and Enginseer
Meelee; Xenarite and Explorator
Heavy Sniper: Xenarite and Dominus.

I heard Xenarite builds are focused in crits, so, which ones I should put in place of it for said builds?

The successful build I got in my first and only gamethrough got advantage of two main points:

-Using the huge area range weapons for obliterating most enemies on first turn, which you can only safely use on first turn because they have friendly fire, so in later turns you would damage your own units
-Close range masteries for picking up the remaining enemies after

There is a mastery which focuses on giving you action points which, if exploited to the maximum, allows you to play 2 turns inside your first turn. This is what I used to buff my first attack to the maximum, using one of the late game huge area ranged weapons.

Then there is the close range mastery, which allows you to move a priest to twice the range inside a single turn, to pick up the remaining enemies which didn't die.

I can install the game again to give you more detail if you need it, I played this game years ago so I can't fullly remember the details.

Also, if you enjoyed the main game, there is no reason you shouldn't buy the expansion because it's more of the (good) same.

You can wait for a sale and then grab the expansion, it's absolutely worth the price in quality and gameplay.