Modeling questions

I have downloaded a Truggy Trailer from Thingiverse. I am working to convert this trailer for a buddy of mine for his use. I get ready to print it and it is not doing too bad. I have 4 double bead lock rims completed. I have 3 parts printed for the rear deck. I need to print the front parts of the deck and some misc parts and suspension parts and I will be mostly there. After that I need to be able to go in and do some serious editing. Problem is I do not know what I am doing. I work in AutoCAD.... I need to take these models and be able to cut parts of them so I can I can print them as the frame and all of the middle parts are too big for my print bed. Once I can cut them up I can print them, Take them to him and find out what we need to edit from there. I know I am going to make a custom ramp that will fold up and down, and Have that already designed in my head so I will need to model it too. Can anyone recommend a good tutorial or something someplace to teach me how to cut parts and all in 123D. If not can anyone recommend something I can download and use that is free?

As soon as I get this completed (It is a paying job) I will be ready to go in and do my Auto Bed Leveling, and the new zstop mods. After that I want to put some lights on this thing so I can see it print.

Right after that I will be doing the homes I bought this for.

Pictures will be posted of the trailer later when I get them cleaned up and enough printed so I can show what it is.

Also I want to take them out to the garage and try a part or two out in my bead blast cabinet. I think if I hit them with a little walnut shells it will really dress up the finish and then I can hit them with some flat black. After that some custom lettering and this trailer will be ready to show off.

Hi, I'm not too familiar with Autocad but I don't think it is the most useful tool to handle meshes. It works great for engineering though, so your new parts should be easy to define in a parametric world.
Meshmixer is free and you can cut your models, but it is not a precise process. It might create issues in your assemblies.
If you're trying to do assemblies and engineering stuff, you should try to work with real solid formats and avoid STLs. See if you can get an IGES or STEP. At least it is something you can work with fine in Autocad.