Work out what the biggest expenses are (ie engine and gearbox), work out if you can afford to replace them if needs be even if you have to drop the cost on a credit card as a temporary measure, and also work out what the most common issues are and how much it would cost to fix them yourself.
Personally I don't think it's worth it on a Zed, but it's a must-have on something like a Porsche where part costs and labour rates are daft money.