Hey!! Welcome back for the time being
They tank is perfectly fine and as you said, you'd just need a heater. I have the 3 gallon cube of this one. But I feel like it would be perfectly fine to use space wise since it's not taller rather than longer...literally a cube haha. That's mostly what I look for in tanks is just longer rather than tall so it gives more space for them to be able to breathe but that tank is certainly sufficient enough
I also use the Whisper 3i on two of my tanks, it's not the best but does well enough. Oh and as far as the cycle goes, you will end up with a cycle regardless because you'll have the filter (unless you decide not to use it) just because you'll inadvertently grow your BB but it won't matter if you keep up with the water changes and all
It's when you don't that it becomes more of a problem.
EDIT: oh yes, I use the Marina C10 on mine, works great and I've had it for nearly a year now with no problems. Of course every heater can malfunction no matter the brand ^^ And the filter yes, is a little strong when you first start up but after about a week or so it calms down like every other filter out there