Hi, and welcome
The size is a perfect choice
Ive used an explorer 1st edition for years (like 10 years !) and I still love it today. I believe the 2nd edition fixed some of the issues with the 1st, so Id think in terms of investment you cant go wrong. They are a little prone to rust over the years, but plenty of people have repainted them, including me, so it is maintainable.
Ive just googled the Little Zoo Trekker, and from a first look what stands out is that the doors are smaller than the front of the cage, meaning you would lose one of the big appeals of the explorer / SRS, which is the unobstructed access for cleaning, and inserting/removing the substrate base.
I have a third suggestion, the Savic Royal Suite (SRS)
Used to be the more expensive cousin of the Explorer, but it seems theyre now more or less on par cost wise, and has the benefit of having horizontal bars on the front and back panels making more options for climbing rats and hanging toys/hammocks (the all vertical bars of the Explorer are one of the drawbacks that persist in the 2nd edition, imo).
If I was starting with a new cage now and its in the same budget range, Id be going for an SRS over the other two.
One thing you will need to question though, is whether the Explorer or SRS are ok for such young uns. Im not certain either way as to bar spacing or potential hazards, but someone should come along with more experience of babies.
Also, you will likely need to make a substrate base at some point to keep bedding from flying everywhere, but theres tons of options there from bespoke metal trays to quick and easy correx designs.