Ooooo, this is a marmite issue. Be careful!
Have a search of the site for Auxialliary and punch-up, that should help.
Some people swear by an extra engine, others cannot see the sense. Both camps feel quite strongly about it too.
Personally I am an auxiliary free boater, but there have been times when I have succumbed.
It all depends on your own boating criteria really. You need a list of essential kit and if you are like me you will not be able to afford everything so you end up deciding which is more important. For example a really good anchor, perhaps a sea anchor too. Spare radio, good flares, perhaps a tool kit for some.
If you do go down the aux route then try and get the same stroke as your main engine, but also make sure you have separate fuel and fuel line for it so that you can mix and match depending on possible fuel problems.
You will also want to take into account the conditions you are likely to encounter, a very small aux is not going to help if you regularly face big tides or areas of chop.