It's always been a year at a time but not sure if COVID-19 has softened their stance. They won't allow rolling and will disconnect you if you don't recontract in for another 12 months. They're pretty forceful in general but the number one outside of London.
In order to join you'll have to be registered with the TPOS or ombudsman etc. Sales or lettings only will be cheaper than sales and lettings. Zoopla only offer sales only or sales and lettings only, no discount for lettings only.
No such thing as pay per listing or pay as you go. On Zoopla you can get a 10 properties max discount or 20 properties max discount, Rightmove might have similar options.
Would be worth seeing if there are two quotes from RM, one for online agents and one for traditional but based out of your brother's house or a local PO box/their accountant etc. With Zoopla traditional agency has a different ratecard compared to online.
Bold time to be starting an agency online or otherwise.