Many repair manuals are available online. Google tradebit.

It also helps to have mechanic friends at your dealership and other dealerships who are open to doing side jobs.

In the last 10 years Ive had to call in the dealer 2 times when there was no other way to access a system. But I explained to the customer why that was. The manufacturer created this situation where only their mechanics could access and set up some systems. And when the customer has to pay the bill from the dealer there's no question who they'll blame for the situation.

And 99% of problems don't need access to proprietary repair platforms. Just keep working and learning until you feel confident that you can handle most anything. Treat your customers fairly and show up as soon as possible and you'll keep them forever.

I had a friend in the business who would always have his customers call the dealer when he went on vacation. When he got back from vacation his customers were more loyal, not less.

You can also pick as your first big customer a place that has trucks whose systems you do have access to. You can get the handsets for GE and Zapi from Flight Systems. Almost all my Hyster and Yale machines are EV100Lx. I've done well with just the GE handset and Curtis software. I keep thinking about getting the Zapi handset but I've never really needed it.