GNS430W is TSO C146a so that will meet your needs. That price is pretty low compared to having to hold for an hour (800 litres of fuel) or divert due weather. Not sure of your installation cost but can run into £1000s. Even then you will save that money pretty quickly.
Higher end FMS's are going to be in the order of £50,000 plus installation. So the savings need to be that much more.