Welcome to the board! :-)
I'm studying soils on my RHS Level 2 at the moment, and one thing I've gathered about sandy soils is that whilst it gives good drainage, and generally warms up quicker than a heavy soil, it can lose moisture and nutrients very quickly. It sounds like you're asking the question at the right time (i.e. before you get started).
You've not mentioned what PH value the soil is, or how big an area you're talking about?
Whilst I can't give you expert advice, I'd be tempted to get a good thick layer of organic material on it now; leaf mulch or good garden/kitchen compost would probably be a good bet. Are you planning on then buying a bulk load of top soil to raise it up to 45cm? 45cm does sound very high; would 30cm be good enough I wonder?
I know you can add powdered clay to extremely sandy soils, but I don't have any personal experience of that, maybe somebody else on here does?
I'm not sure as to the timing, but there are also green manures available quite cheaply nowadays, such as winter rye, that can also be planted to help fix nitrogens in the soil, etc. With such a mild winter at the moment, it's not too late to sow these crops (I did just 10 days ago - and I'm in the cold northern area of Yorkshire, and they've already come on a treat).
Anyway, that's just my layman's thoughts; I'd also be interested to hear other people's opinions who are more experienced than myself.
Cheers for now,