Some quick thoughts:
Printing your own parts is a great idea if you want to have a new hobby and have enough time on your hands to deal with the foibles of hobby-level 3D printers. But If you’re tight on time and get frustrated when things don’t work perfectly the first time, but have money to spare it can be easier to just buy parts from Shapeways. You might also look at https://www.3dhubs.com/ to find a local person who can 3D print stuff for you, perhaps at a lower cost. There may also be a makerspace in your area that you could utilize.
If you are going to purchase a printer, something to highly consider is printer quality. We recently purchased a Lulzbot Taz 6 because it is a very well reviewed printer that is also 100% open-source. We also have a Form 2 SLA printer for ultra high-resolution stuff. However both are quite expensive. But you get what you pay for a lot of the times with these things. The nice thing about having a printer (as mentioned) is that you will be able to continuously upgrade your FarmBot as we release new and improved designs. You can also augment your bot yourself with new tools, etc. But again, just consider whether or not you have the time for this stuff or if you would rather just buy parts.
As far as 3D printing the plates - you could do it, but you would definitely want to modify their designs to have ribs and thicker sections. FarmBot gets most of its rigidity from the plates, so you don’t want them flexing much or deforming under the stresses.