Well, the first advice would be read up what all the members of the TEFL.net forums and the resident specialists say on the subject. That will probably answer most of your questions. In short however, CELTA is a TEFL/TESOL certification. It's just one of the many. However, you are probably right in assuming that it is one of the most widely recognised and its standard is recognised as being one of the highest. It is indeed also one of the most expensive.
If you have a language or linguistics related degree, and hang a PGCE onto that, you would hardly be expected to provide a TEFL certification as well - it would be kind of redundant and an unnecessary expense. It takes much, much longer to do a PGCE however, as I'm sure you know already.
If you are only interested in part-time work, you may wish to look into the earnings potential and weigh up how long it will take you to amortize the cost of getting qualified.