As the first respondent to the OP let me be clear. There is nothing intrinsically wrong with using film professionally, for some projects it can lend an aesthetic advantage that adds value which converts into new clients and hard cash. My alarm bells rang when ongakublue said, "last week I sold off all my digital cameras", followed by "my question is about colour film. I have only shot b/w so far", and "I can't afford and don't have time to do all the processing and printing/scanning myself. I must use the local photolab". Taken as a whole, I think we can be excused a certain incredulity, even while our sympathy remains undiminished.
When film was the only thing photographer's used, most large towns had fast turnaround professional labs who serviced the wedding and portrait businesses with high quality processing and client proofs. Unless you're very fortunate, the local photolab does not offer anything like that service. You might get a disc, I have no idea if that's okay with your client, or what plans you have if they want large prints. If you only "think" you'll get negatives back you need to make sure quickly. None of those issues would be a problem if you kept a digital camera and introduced film alongside.
We all have to start somewhere, my first job was a catwalk show with an OM1, 50mm lens and a very underpowered flash, but I came through it somehow. I hope you survive the shoot and you and the client are happy with the results. Professional photography is certainly not all about gear and technique, you may have a human touch that shines through in the images, but I would certainly shoot a test roll of a friend and give the lab a run through with your chosen film. Good luck!