No ventilation is a comfort feature, not needed with that budget. Rotex is a cheapie and quite loud. I would be careful with the installation location.
We have a Rotex and are completely satisfied. It is not loud.
Personally, I would never want to do without ventilation. Whether window rebate ventilation, a ventilation system, or other techniques. From my point of view, it not only provides comfort but also protects my interior space.
With airtight houses, you simply can’t keep up with ventilating by opening windows anymore. Whether mold can develop depends very much on individual conditions (large wall units, aquariums, intensive showering habits with an open shower, ...)