Since this is a first-time experience i will not buy some expensive (but
surely VERY useful) system (like WEBASTO). It could even be the last time,
so I was trying to do it cheap.

An Origo "Heat-Pal" alcohol heater will heat things up safely,
but it will produce a lot of condensation and some people don't
like the smell. A small propane catalytic heater may also work
for you. Just make sure it's certified for indoor use and shut it
off before you go to sleep. Both of these heaters will use up
oxygen so make sure you have plenty of ventilation while they're
burning. Portable heaters can be a real fire and burn hazard.
Also make sure you burn off any shipping oil before you put it on
the boat. Be very careful and you should be fine.