You can use 23mm tyres as long as the rim is narrow enough to support them. A rim might be narrow enough for 28s but 23s may cause problems. In sort you won't know unless you can check the rims.
A for advantages/disadvantages that really depens on the precise brand and model of tyre.
Personally I know for a fact that 28mm tyres are widely available in shops which means you can buy them now. If you're going on saturday I'd already say it is too late to buy on-line and be confident of receiving the package in time.
Otherwise, why not just buy them in the Netherlands? You think they (of all people) wouldn't have bike shops?????