A good starting point is speedmaster101, but in my opinion the Speedy market has softened quite a bit recently so you have to be realistic in your expectations.
https://speedmaster101.com/price-chart-2/
And yes is a transitional model with the step dial and applied logo, which is actually not so common and much more desirable than the later model with the flat dial. The only obvious problem is the lume which isn’t great, but overall it looks pretty good. An STS service, even if it was done years ago, is also a plus. Pinning down a figure is hard. I had an agreed purchase in the spring at €4K but that would’ve been a bit of a steal, and also it had some issues that would have cost €500 to put right. It didn’t have the 220 bezel although the market doesn’t really seem to value that. Maybe you should be thinking in the £5-6k range, but as I said the market seems very slow so finding your buyer might take a while.