Electric showers used to last at least a good ten years and I have replaced many that had lasted upwards of twenty years. More recently however, I have had to replace cheap showers which have failed after only three to five years.
Like everything else, manufacturers are concerned about profits and you gets what you pays for.
8.5kW is actually fine for the vast majority of users - I don't understand what pleasure some people (women mostly, as it happens) get out of standing in high pressure jets of scalding water for hours on end - most see a shower as a utilitarian device for efficient personal hygiene.
So, for my money, buying a good quality 8.5kW shower will probably get you more long-term reliability than buying the cheapest, higher-powered shower.
But, as others have intimated, depending on the condition of your incoming electrical supply and the relative location of your shower, you should expect to spend several times the cost of the unit on having a safe and compliant installation.
