I'm pretty sure that the Robertson AP 35 will also work depending on which junction box you have.
The J3000 box is the one that works with both. You should check with Simrad since my recollection may be incorrect.
I installed the AP35 in my boat 19 years ago and the AP45 screen was an upgrade at the time. The AP 35 and 45 were Simrads Commercial line of autopilot and were sold for a long time without change. Used control heads come up for sale on eBay often. I see more AP35 for sale than the 45. There are two AP35 on eBay now for around $1,000