If you post a manufacturer, make and model I may be able to give you some feedback. You usually get what you pay for in a sewer machine and the risk of breaking a cable or getting it stuck in a sewer has a number of variables involved. Most beginners that I know use a sectional type of machine. Once you get some experience and a feel for how the cables are responding in the pipes maybe consider a drum with a continuous feed cable. They usually take a little more skill to be effective and to avoid breaking or getting it stuck.
Advice to a beginner...always warn the customer that there is a risk of breaking the cable or getting it stuck and that you cannot be held responsible. You are there for a problem inside the pipes and you don't know what it is and if the situation worsens they may need more work. Get that in writing if you can and signed.
Make sure that you are insured for any possible mishaps, damages or injuries.
Charge accordingly for your work. Most people today expect you to clean up any mess that was made by you...not necessarily all of the sewage that backed up, but a reasonable clean-up. Commercial jobs can get really messy.
Get in touch with a local flood/restoration company and carry their cards with you. When the client says "what do I do about all of this damage/sewage/mold, etc?" you have somebody to refer them to. It shows you are prepared and care about them. They will call you and refer you for repeat business.
Nobody ever sat down and gave me the 1,2,3's of drain cleaning. I was 'self taught', I guess.
I have compiled a list in my brain of all the mistakes and bad things that have happened to me on sewer jobs over the years. Hopefully you don't have to learn the hard way like I did.lol.
It's good to ask questions, every day an opportunity to learn something new is presented to us.