Restrictions in contract - could this be a deal breaker?

[Millionairerow]

Got a new job. Awaiting contract. Going to a direct competitor but I have restrictions in my current contract about not working for same clients for 12 months post termination. So due to have a call with hiring managers to see if there is any overlap of clients. They've advised I get independent advice on it but my question is

1 why do I need independent advice?

1. Is this a potential deal breaker
2. When is it best to hand notice in? They say it's not going to delay them sending out the contract etc but I'm under pressure to put in info for references and background check by Friday.

I have a friend who will look at the restrictions for me but she says they're not usually enforceable. MY main worry is me handing in my notice and being left high and dry if these restrictions cause my new employer to back out. Help - clock is ticking.

1. I expect they are advising you to get advice so you can find out whether the restrictions are enforceable. "Not usually" is a bit of a stretch - it depends how they are worded and on whether they are reasonable in relation to your specific role and circumstances.
2. No one here can answer that. It sounds as though your new company don't think so if they say it's not going to delay them sending out the contract. If there are overlapping clients and it's crucial you be able to work with them and/or they are expecting you to poach clients then yes it might be a dealbreaker but in that case why would they bother carrying on with the contract issuing process before finding out?
3. I would never advise anyone to hand in their notice before receiving a contract. I can't see any reason to hand in your notice before you've received a contract and have had it confirmed that there are either no overlapping clients or they don't mind you not working with them.