So, you’re about to start out towards an exciting new career; it sounds great.
Starting with your third question about the difference between TEFL and CELTA. TEFL stands for teaching English as a foreign language; it’s a subject that is studied by people who want to teach English. You can study for a certificate in TEFL, a diploma in TEFL and a masters in TEFL, amongst other things. CELTA is the Certificate in English Language Teaching to Adults; it used to be called CTEFLA: certificate in teaching English as a foreign language to adults. It is validated by an external body. Basically, on a CELTA course you learn TEFL.
It can be very confusing when you are new to the field; especially as some people use CELTA and TEFL interchangeably. CELTA is the qualification, TEFL is the subject. If you think of them as degree and psychology (degree is the qualification and psychology is the subject), you will understand the difference.
To find a good TEFL course, I suggest you look at our TEFL course database or check with your local British Council. As there is such a wide choice, it’s important to ask lots of questions. Take a look in the teacher training forum, there are some very good tips there about what to consider when choosing a course. An important point to consider is how many hours of observed teaching you will do. I’ve never heard of the ACE certificate of Manchester University; so, I am unable to comment on the reputation of the certificate.
As for your level of English, I suggest you check what other training centres say. Many courses take non-native speakers if they can prove their level is high enough to be able to cope with the content of the course. I think it is wisest to check before signing up for another course in English language.