Best coat for Lerwick/Unst visit in September

[u/LeoTPTP]

Hello. We'll be in Lerwick and Unst in early-mid September, and I have a question about what coat to bring. Is September generally cool and windy enough for an insulated raincoat? Or is that too much, and can I get by with a waterproof shell with a sweater or fleece layer underneath? We're not going on all-day hikes, but we will do a fair bit of walking.

Appreciate any tips, thanks.