Sorry but got to disagree. Building sand is for bricklaying and is too soft for rendering. Use plastering sand and cement. The bags of ready mixed mortar is more than likely made for laying bricks rather than rendering, read what it say's it is suitable for on the bag. There is also a lot more to consider other than the sand and cement such as what strength to mix it and what admixtures to use.
The pre mix in various colours sounds more like scatch render. You will need some expert advice on using that particular material as some of it is made for machine application only. The application is completely different to conventional render and is an expensive material to purchase. I had a job done in this material a few years ago on a new build project when I was a property developer. The guy I used can be found at www.rendermaster.co.uk
and the material in question may be found there. He's been rendering Sheffield homes for the last 30 years as I know of. Going back to the subject of plastering sand without some kind of plasticiser you may find the sand difficult to use. As for life expectancy well it depends how it is done. Go up to the West side of the City and you will find houses that was built in the 30's that still have there original rendering on. The right preparation, materials and conditions there is no reason it should not last for at least 30 years. What I mean by conditions is of course the weather at the time and not so much by the bad weather but for example the recent hot weather. Rapid drying can cause failure far easier than the cold weather. Spring and Autumn are good safe times as far as temperature is concerned.
Good luck with the rendering mikomi and hope it goes well. Pagets and Hardcores are good suppliers for the materials you will need.