I'll try to be as nice about this as I can. This is a really bad idea. Just because someone is asking you to do something, does not mean you should do it. The market is already full of SEOs that have no idea what they are doing. It doesn't need one more.
Spend a solid year or two learning how to rank your own webpages first before you even think of touching anyone else's business. You can do some serious harm to a business if you do not know what you are doing. When you take on a client, you hold their business in your hands, and on the online side of it, you can completely annihilate it.
If you are going to do a site audit, probably the best tool out there for it is Screaming Frog. It's reporting kind of sucks, so you have to know how to organize the data and present it. You also have to know which data is useful and which isn't. It does a solid job of crawling sites though and grabbing what you need.
