Hello There!
Well this site should help you compare between Montenegro and the UK:
http://www.numbeo.com/cost-of-living/co … Montenegro
It's very hard to tell you for sure if 600€ is going to be enough or not. The minimum salary here is about 200€, the average salary is around 450€... Some people actually manage to live with 450€, but I suppose a lot of them don't have to pay for a rent for example (owning their home, or living with parents/family...). Many people also have this minimum salary + some extra money from other jobs (or from renting part of their home) etc.
If your job will give you some extras (tips...) then maybe you'll do fine? Many waiters here are paid 300-400€ but get almost as much in tips.
Also, as Nemanja mentions, you might not be able to go out a lot, even though drinks are cheap.
About rentals, the prices Elisabeth gave you are pretty much correct (I'm quite well placed to tell you about this as I run the only agency for long-term lets in Montenegro), and it is true you need a good notarized contract to make sure you can stay over the season. But not all owners will ask you to pay 12 months in advance, as long as you sign a notarized contract and usually if you pay c. 2 months security deposit which you would loose if you leave the flat before the term.
I hope this helps,
Carine