I am using a converted TronXY X5SA 500 Pro for this effort. It is a large coreXY printer with a 500x500x600 build volume. I have multiple printers and I have the ability to design and print my own upgrade and custom parts. Which is great practice for the rapid prototyping. And when I think I need a better designed part I just draw that up instead of hoping someone else might have designed exactly what I need or accept a compromise from the list of things I can find. To be efficient prototyping with the FDM printers you should have a small form factor printer that is just reliable and works. In past years I had a Monoprice select mini printer for this with just a 120x120x120 build volume. But most parts I made for the bigger printers always fit nicely on the build plate. Currently I have this job filled by my old Geeetech i3 Pro B I have upgraded a bunch: GEEETech i3 32bit Rework (3dprintboard.com) .

And if you want to see my Tronxy X5SA 500 Pro and how I have reworked that on it's initial assembly that thread is right here: My First CoreXY (3dprintboard.com) .

If you take this path then overcoming the failures of any printer design shrinks and you can see these robots for what they are. A build volume and your skills. Get the printer with the build volume and shape you like. Then identify the good parts. Stay away from knockoffs. These printers all suck because they are made from the cheapest parts money can buy. So to replace the knockoff parts with knockoff parts just wastes your time. Going down this path will put you on a better footing for rapid prototyping. And likewise if this scares or intimidates you what does that say about you prototyping auto parts or anything else? Just a thought, there. Good luck.

And a final thought on the build volume. If you need actual 300x300x300 you should look for 400x400x400. When we print at the edges of our bed sometimes we get the lifting and we cant print the brim and sometimes, SOMETIMES we don't even get the actual advertised build volume out of the printer once it is assembled. If you have to build max dimensions it is important to be able to. But we shouldn't plan our printer size on building the max build volume regularly. It can only make us struggle more for the good quality and bed adhesion. What you want to print should comfortably fit on build plate. And nozzle diameter and layer heights should increase with printer size.