Hey man, welcome to the forums. I am newer here as well but have worked in a ski shop for a few years.
The way I look at your dilemma especially if it was a free board is ride it untill you can scrape up some extra cash to get a good set up. Things do become a bit pricy once you start adding it up. If you were to come into our rental shop, i would throw you on around a 155( seeing we only have 150/155/160's available. I am 185 lbs 5'9''and ride a 156. Which is somewhat similar to your european conversions. Another idea about buying your own set up is get it from the states. i have had many european people come in and buy complete set ups from us and save money because the American dollar worth has turned into the ditch. Keep the board, ride the shit out of it. and then step it up. If someone is looking to buy a board, just starting off usually they go for what's popular such as a burton, or ride rather than something off the net.
Hope that helped. onelove