Personally, I have seen some people that were very good in their Masters totally screw up their PhD. And other people are not doing very good in the Masters but doing a great PhD after and get an academic position. A PhD is long time, so be sure that you like the subject, be sure that you can publish within the subject. You have to be comfortable with the people you will working with. If you can work with the people that will offer you a PhD before starting the PhD, that is a very good point. That maybe out of the question but even after 6 months or one year, if you see that not doing it, that better to leave and try to find another PhD. You can do well on your second PhD and get an academic position, I know someone who has done this.
If you don't find a PhD that you feel is good for you it's definitely better to wait. Going back to work or doing an other Master, but in a related field. That gives you more experience and more time to find a good PhD subject.
When you start your PhD it's kind to be like in a tunnel, sometime you never really see the end. I think most of people get discouraged at one time in their PhD, even the better, you really need to believe in yourself.
The main problem by doing a PhD after a not so good Master is that you will have more difficulty to find a good PhD subject. You can definitely find one but you have to find a good one for you. If nobody wants a PhD subject, there is a reason. That's not mean that it's bad subject but you have to figure out why it's less attractive.
Some questions that may help you:
Did you mentor have previous PhD candidates and how did those PhD candidates end up? Generally when there is not a full transparency from the mentor there is a problem.
Are you comfortable with your mentor and the team that you will working with (if there is a team)? Can you talk freely? If it's not comfortable when starting I don't think it's will going better with the stress of the PhD.
Is the plan of action of the PhD well defined?
When can you expect the first publication? For example if the mentor said you that there is a publication in progress and they can add your name to it that a good point.
Do you feel comfortable with the subject or does it look too hard? Especially if you have to write a program or build an experiment. You have to estimate the time that it will take you and if you can get publication from this. Some mentors don't hesitate to take a PhD candidate just for coding as cheap workforce.
Hope it helps, good luck!