Thanks Landlockedpirate. I knew I could count on your wise opinion.
I saw the rig today and it was basically brand new. It was bought new by some old man and he didn't like his first experience at sea and kept it in the garage ever since. Then sold it to another guy witch had plans for it but ended up never using the boat since he has a bigger boat.
The boat is indeed a 380 and the price is 2.650€ (£2120) with the trailer (that has never seen salt water in his life). It's nearly half the price of the other Quicksilver HD I found and on third of it's value brand new.
As for going bigger in the future, you're probably right. It took me just one month to think about something bigger...
The thing is, in Portugal 25 hp is the maximum power you can have without having to pay taxes yearly. 30 hp costs 60€ per year and 100 hp costs 200€. I already have to pay insurance (70€) and I would like to keep its costs as low as possible. Staying at 25 hp is the only way possible. Therefore I can't go much bigger, maybe 4.20 m at max for 25 hp.