Hi Steve
The flow rate/temperature of a shower is determined entirely by the physics - all 9kw showers will, by definition, deliver the same balance. So to get higher performance, you have to up the kilowatt rating, which leads to potential upgrades to the electrical installation, particularly to the cable.
Its not clear from your post whether the whole installation is new (in which case using the correct cable should be relatively easy) or just a new shower to replace an existing one. If its the latter, then you need to check the cable size already installed - that will be your limiting factor.
As for longevity, there are two factors - make, and nature of water supply. A Mira Sport is not the cheapest, but should give long service, but if you are in a very hard water area, then any electric shower is likely to have a limited lifespan unless you take steps to treat the water supply.