How much "leaking" is acceptable? + some general descaling questions

Long story short: finally found a Silvia (V5) second hand for 250€. Thankfully traveling by train is dirty cheap here in Austria (if you have the right ticket of course) so traveling around 600-700km for the Silvia nearly cost me nothing, except 6 hours of my life.
Got there and the sellers were a young family who is moving. They told me that they only used the Silvia a couple of times, because they quickly found out that getting an Espressomachine is getting a new hobby and there is no time for that. Now they are moving and thus its the perfect time to get rid of the machine.
Back at home, I took of some panels and this machine really looks good inside, at least to my eye. However they must have had a very high calcium concentration there, because there is limescale visible even on the rubber tubes for the water tank, no idea how the machine looks inside the boiler for example!
First thing I did was to clean the group head, disassembled it, cleaned everything and ordered a new jet breaker, screw and shower head, to get rid of this crazy screw.
Today I took my blind basked, some Cafiza and did some back flushing. Everything looked very clear, which would explain that this machine really only saw a few uses. However I noticed something strange: there was some water buildup under the drip tray. Took some panels off but could not find any leaks. I read on here, that the solenoid outlet could spray quite a bit if limescale builds up there. The other potential leak could be the connecting hose from the pump to the boiler (I only took off the top and front panel, have no time, should be studying for uni :D) but from the visible inspection, I saw no waterdrops on there as well.

Is water build up under the drip tray normal?

Now to my second question: Tomorrow I want to descale the machine. Bought some Durgol Swiss Espresso descaler in the supermarket. Is this stuff good? According to Gemini, you should not use citrus based descalers as they can attack some internals of the machine. The Durgol is based on sulfaminacid which is better (according to Gemini). Gemini also told me, to NOT backflush the descaler, however I saw a video, where they specifically mentioned, that backflushing with descaler really descales the whole machine, which could solve my solenoid outlet spraying maybe. I also read that you run the risk of big parts of limescale getting stuck inside the machine if alot of limescale has build up. However, if the machine really got used only a couple times, even the highest concentrated calcium water couldn't produce this much limescale that I run this risk, or could I?
Do you have any specific tips for using Durgol or should I buy some other descaler? Any input is highly appreciated :)
Sorry for this long text but as this is my first espresso machine (coming from the Flair 2 Pro, Moka Pots and V60's) I want to do everything correctly, especially on my frist attempt :)

Added some pictures of the machine as well :)