I would recommend getting the Complete edition because it gets all expansions up to the current one. Make sure it's for the correct platform. If the trial version is the PC version, expansions should be the PC version too and if the trial version is the Steam version, all expansions should be the Steam version too.

You should receive a code from the retailer by email which you can put into your mog station account (http://sqex.to/Msp) to register that you bought it.

You already have an account and the launcher because you have played the trial, so the launcher will download anything that needs downloading.

You can subscribe on the mog station as well http://sqex.to/Msp