100 euros a year for engine maintenance is nowhere near enough unless you maintain the motor yourself and even then, that will only just cover the materials.
This again will depend on the motor. If you buy a Honda/Suzuki/Mariner 2.5 hp and potter around for a few hours a year, and you do the servicing yourself, your €100 per year should be OK.
It's a 4.2m planing hull. It's going to need at least 25hp to plane with more than one person aboard. 40hp would be better.100 euros will only just buy an impeller, gear oil, gearbox fibre washers,grease,fogging oil for the winter layup, fuel stabiliser for the same,and anti corrosion spray to stop the powerhead turning into a pile of rust. Add 40-50 euros for a yearly anode change (if you're leaving it in the water, you'll need to). That's without the labour at £25 to £50 (pounds not euros) per hour for the work, unless you can do it yourself.
Well, this rather depends on the OP's intended use surely? Perhaps the OP can clarify this?