Great advice that thankfully I learned before starting my build, working on cars for High Performance Driver Education (HPDE). Bad timing on my quote though as it looks like Pat has passed away recently. :/ In reading backwards (down to page 44 thus far), Pat was damn near a philanthropist from all the time he spent responding to questions here.The few things I can tell you are:
1. Clean it up good so you can see what you've got, but be very careful when poking around seats, cracks, cabinet and compartment doors, and anyplace else where needles can fall into and hide.
2. Once you have it cleaned up, take lots and lots of pictures of every square inch of it, and from every angle. Don't forget the compartments inside also. You will thank me later.
3. Once you think you have figured out where you want everything to go, and get ready to start ripping out wiring, cabinets, and walls, STOP and think about it some more. After that, wait a while and then think about it some more. Considering you are working with a very limited amount of space, you'll find there will be several things you want in there, that will be competing for the same space. It's going to be like putting together a jigsaw puzzle, and once you remove parts and throw them away, there is no do-overs.
I'm not trying to scare you here, I'm just saying don't be in too big of a rush to do something, and then regret it later because your plans changed. Just make sure you have thought it out thoroughly, talk to other people who have already been there, wait to see if anything changes, and then start cutting and removing things if you need to. Most of the wiring circuits for ambulance items you wont need like the suction pump, can be re-assigned to another item like the water pump, etc. I would not remove any unused wiring, just cap it off and label it as to what was used for to make identification for use later easier.
Now, jump in and start planning your dream vehicle!
I'll post a picture of mine here and start a build thread shortly. I've lurked since November 2017, about time I started contributing.