With 3D printing, you're really only limited to designs you can download online and anything you design yourself. Basically like modeling! The only restriction is The quality of your printer and how well it can print.
Most consumer grade printers (the printers you can find in Walmart or home Depot or, basically, anywhere for cheap) will not produce models of "high enough quality" to be used for scale modeling. They are typically FDM/FFF/extrusion/a-whole-host-of-other-name printers. These printers heat up a filament of plastic to near melting point and extrude it through a small nozzle onto a print bed (that is sometimes heated so watch your hands). They have large layer heights of typically 0.1mm and higher. This is not ideal for modeling work. The detail achieved with these printers is also not very good. For various everyday and general printing needs, these types of printers are fine. They are very inexpensive, the filament is inexpensive and they're great to starter printers.
If you wanted to print for use in modeling, the best printers you could get (as a consumer) would be DLP/SLA (digital light projection/stereolithography) resin printers. These types of printers are very similar. The basic mechanics are the same and the one general difference is the method for curing the print material (UV sensitive resin). DLP printers use a high resolution projecter to project a thin 2D slice of the model on to a vat of resin from the bottom. A print bed then rises up and the process starts over. SLA printers use a laser beam instead of a projector. Both are very high resolution printers able to resolve details down to 0.05mm or 50 microns with layer heights of 0.010mm (10 microns) in some higher end printers. Most consumer DLP/SLA printers you will find will generally have a max layer height of 0.025mm (25 microns) and detail resolution around 0.05-0.1mm. These printers print slowly, resins can get expensive and the printers themselves start out expensive (compared to the FDM printers). They typically also require more maintenance than FDM printers. But these are still the progress you would want to get to print for modeling.
Now CAD software is a bit of a different story. There's so many options out there it can be (and is) confusing. Many are very good CAM solutions. Some, not so much. If you're rolling in money and have a mildly powerful computer, I'd say to go the Solidworks route. I can also recommend AutoDesk Inventor as a very good program but both these options have a good learning curve to them. They take a while to really learn how to use them effectively (truth be told, I've never use Solidworks, too expensive for me to even try, but I have used Inventor and generally like it but not too much). I can't say anything about Google Sketchup because I've never used it either. The one program that I can really recommend is AutoDesk Fusion 360. It's a relatively simple program but it's very powerful (and free for students, hobbyists and startups). If you're a beginner, I find, it's simplier to learn than Inventor (but not as powerful) but can still do almost everything you would need. It is a partially cloud-based program though so simulation, some rendering and most project storage is done on the cloud. Storage, though, isn't completely cloud-based. Older projects that you haven't worked on are stored as well as the current project. But the current project is also stored on your own computer as well. All modeling is done on your computer. You can also export direct to STL (one of the file types that all printers read and need to use to make a print) from the program. The user interface is also simplier than Inventor's (and, from what I've seen, Solidworks' too). For overall bang for your buck, I would really recommend Fusion 360. I actually use it nearly everyday making designs for people.