Having once been in the same shoes as you with a new old and not working wrecker I would suggest the following:
Focus on safety and operational items first. All cosmetic items are low priority.
In order of priority
1.) Brakes
1a.) Steering
2.) Engine fluids, filters, belts
3.) Fuel System
4.) Axles, bearings, grease etc. Tires
After you have dealt with the mechanical items then worry about anything else.
ps. by my estimates that would be about 3-6 months of work playing around after work etc. Start by ...
1.) Reading the TM's on brake systems then track all components
2.) Go in TM to troubleshooting guide and follow along to identify each part they call out in a troubleshooting situation.
3.) Then go to the maintenance TM related to the item and perform the maintenance.
4.) Ask allot of intelligent questions on here
Once you have done this for the 4 items above you will be confident in the truck and yourself.
pps. This is my opinion