This might be handy for you
You’ll definitely have space for 3 odom wheels with a 280rpm drive (I was able to fit 2 in a 25x25 base). Just start designing the base in a cad program with the requirements you want (280rpm, 30 holes etc.), and start to figure out spacing for things like odom wheel. Mount the motor on a cut 5 wide c channel rather than a plate
There are free cad programs like fusion 360 or inventor which you can use, so you should be able to get these and cad. If your computer is too bad to run a cad program, try out onshape which is done over the internet