4’x3’ on the LowRider would be crazy rigid, long direction the table and short direction the rails. Same dimensions in the MPCNC would be at the extreme end of sizes for it really and you would need the CAM to be on point.
As for the parts I could easily sell you a set and replacements if needed. I have just kind of been watching and listening to the feedback form the few of us that have one in case we needed some changes. I have been prepping to sell the cut and printed parts so if you want to go that way I can make it happen. I have just been using my free time to work on the site lately.
The LowRider CNC is more purpose built for large sheet material, plotting, and laser would be easy to add, well anything really but I just don’t see it as a practical 3D Printer but I am sure it will happen soon.
The only thing so far that might be improved is the vacuum mounting, but with a cnc router you could easily cut your own replacement/upgrade part.
I have not put too much thought into a newcomer starting with the LowRider, but it seems very similar in build and use to me. Jeffeb3, Has the most recent build and it did have some issues and we aren’t really sure why.
I wish I could give you a more solid answer, but either would be fun, if you are going to be cutting larger sheet materials you probably should go with the LowRider, if you want to experiment and fiddle around you, some 3D printing, some milling, you might want to build a smaller MPCNC and use it to make parts to build the Lowrider, by the time you got through that you would be a pro.