Ok. Will Spark give me a freebie? Or, if I have to buy one, which model?

https://www.spark.co.nz/shop/internet/plans/fibre 

Fibre & router pricing.

Your existing router should be plug and play, the Fibre installer will hopefully plug it in, too. If they don't, you just need to plug LAN1 from your Fibre ONT into WAN (the blue port) on your Huawei device. If you do want a newer device, the Smart Modem 3 is available for $150, or you can do an interest free purchase on it to spread the cost over your monthly bill. If you need more switch ports, switches are very affordable these days, you can pick up an 8 port Gigabit switch for under $50.

If you go to the Smart Modem, you can also use Spark Smart Mesh 2 to extend your range if required., by installing it in another room (preferably hard wired).

Sounds like the HG659b will do for now. Thanks everyone 👍