This is cheap… needs a service? 611 seamaster 600

It’s cheap but it’s not a good buy imo. For a couple of reasons.
1. The dial has too much patina which isn’t attractive. These aren’t hard to find with good dials.
2. It needs a service so budget minimum £225.
3. It’s not the full sized 34mm but the smaller 32.5mm.

So, if the seller accepts £275, you’re another £225 for service. After that you’ve spent £500 on a 32mm watch with a very average dial.
Better to wait and/or pay more for a better one.

Exactly! Patience is the key. It usually takes months if not years to find a good vintage watch you like. Also, don't neglect the sales forums here. Both the Private Sales and Dealer Sales. Most of the dealers are selling modern watches, but some like @nanjingcigaretes have a number nice vintage ones at reasonable prices.

If you like this reference as @Davidt pointed out they come in 2 sizes, 34 mm standard, and 32.5 mm small, or boys size. While that may seem like trivial difference the small size is really small and suitable for ladies or small wrist size. Usually the inside Caseback info shows this , the 34 mm 136.011, and the 32.5 mm 136.012.
But you didn’t show inside Caseback so @Davidt noticed that movement doesn’t have spacer ring so it must be smaller size.
Well spotted indeed! Experienced eyes like this make OF great. ( I missed it)
Here is movement of the larger 34 mm watch showing the spacer ring (red arrow) compare to your photo.