Hi! Welcome to HFGF!
I agree with the concern the Zen DAC doesn’t have the power to get the most out of the Sundaras. Getting a dedicated power supply for it doesn’t actually increase the available power, either, it stabilizes it. USB power can be a bit dirty in that it may not always be a consistent, full, 5V. I still like the Zen DAC as a dac and agree that it’s bass boost is nice. I have more thoughts on it here. The Sundara is one of those headphones that will likely benefit from that boost but it’s gonna require some power. I don’t know where in the EU you are, but can you get ahold of any of the JDS Labs Atom amp, Monolith Liquid Spark amp, or either of the Schiit Magni 3+ or Heresy (this one especially might be a challenge)? Those all deliver plenty of power to drive the Sundara’s using a Zen DAC as a DAC only and with that bass boost enabled. Otherwise, for EU listeners we tend to recommend Lake People amps a lot as they are built in Germany. Their entry-level model is the G103-s. You might consider going in stages here. Buy Sundara + Zen DAC to start (go ahead and get a PSU for it, but all you should really need is a quality phone-charger wallwart and a USB-to-DC power cord - which I think iFi puts a short one in the box so you might just need the wallwart and a USB extension cable). It will be decent. Then, let funds build up again to grab the G103-s. That will rock your world.
Good luck!