Late to party, but I converted cable steering to hydraulic on my Willard 30 when I owned her. Cable steering was a jacketed cable, similar to outboard steering. Reason for conversion is similar to OPs question - to fit a better autopilot (boat didn't have one when I ourchased).
In today's pricing, Capilano helm pumps are over $1k each. I'd guess the hi-pressure tubing is $4/foot (I could be way off). The end fittings are probably $10/ea. And there are some oddball installation items yjat always come up, including oil. On a 42-foot boat, I'd guess parts would be in the $3500-$4000 range, plus the autopilot. If I remember, deinstall of old and install of new was around three full days, and that was 20-years and 40 lbs ago. It was not technically difficult, just time-consuming. Including the AP, I could easily see the price tag running to $10k or more if professionally installed at a yard.
If I did it again, I would use a bench vice to attach the reusable flare fittings. Pair of wrenches was a PITA and slow.
Guessing there aren't too many 42 foot trawlers that don't have autopilot already. The boats that don't already have it are probably older, poorly maintained ones that would not have a positive business case to install one. Exception may be Midwest time capsule boats that are warehouse stored half their lives and sometimes have had minimal upgrades over the years. Literal "barn finds"
Peter
