You could modify the Aixun too. The power supply just goes to a 2 pin connector on the front controller board. You can remove the power supply board entirely, run 2 wires from the control board to barrel jack on the back (probably 2.2mm or 2.5mmm), then use your bench supply to power it. Just make sure the polarity is correct. But honestly, the power supply seems to be acceptable. For as long as this thread has been going, no one has reported any failures. Just one cracked inductor that may have already been damaged before installation (quality control doesn't seem to be Aixun's strong suit). I've already said my peace on the soldering quality inside these units, it seems to vary quite a bit, but again no failures. For anything that imitates quality, industry-standard gear, at a fraction of the price, there are always some concerns/issues. And the majority of them can be corrected. That's the great thing about the EEVblog forums! If there's a little solder clean up, I can live with that.
As for the cost of the Aixun, 130usd/112eur might be a little steep if you don't want to use the power supply. But you can also just buy the base unit (no stand, handle, or tips) for 85usd/74eur. It will use the T12 as well, but you will have to change the wiring in the handle, and probably remove the motion sensor. So you can still use your t12 tips and handle, and get a 245 handle and tips at a later point. As for the 245 tips, where are you going to buy them from? 50 euro each is really expensive. On TME, the common 245 tips are 24usd/20eur, while the long life tips are 38usd/33eur. Not sure what VAT would be, but it shouldn't double the price. Unless VAT is 100%...