Depending on your design, powering the back wheels could be more effective.
If someone gets under you (absolutely possibly, even if you try to stay low to the ground) front wheels are off the ground almost instantly, so you don't have any control left. Depending on size, positioning etc. you might still have back wheels on the ground, allowing you to drive backwards down from your enemy before you get shoved into the pit or such.

Anyway, 5s battery for 18v motors is what is mostly done, i think. the motors should have no problem to handle that, at least not with the right gearing (which you will have on drill gearboxes and standard sized wheels. just don't use absurdly bih wheels like i did and land on the side...^^ remembers me i should upgrade my build diary)
I don't know what motors exactly you are using, but 30A botblitz per side for drill motors sound okay, but only just.
If you gonna push a lot (stall amps are way higher than usual amps) they might get hot, but i didn't use them, so better wait for someone who did.

The main question for me here is: do you want to put this practice bot, after it's done its job into a shelf or such, or do you want to reuse (internal) parts for a "real" bot later?

first option: take the cheapest parts you can, even if you drive on the street and try to do tests, those tests will likely be softer than the arena, so no big risk to burn anything. and even if: it was just practise.

second option: take something good right away, better buy good once than to buy cheap a bazillion times.
At least on the esc and battery, which will likely be the most expensive reusable parts in your first bot.