lol spammer - he's gone :-)

I plan to (Hopefully) replace the main board entirely at some point in the near future. I have no idea about main boards for 3D printers
So why do you want to replace a board you have yet to even try ?

An awful lot of issues we get round here are precisely that - people chopping and changing the electronics on printers with no actual rhyme or reason behind it.

Build it, try it - if it doesn't work - THEN maybe think about changing things.

Why would you want to upload marlin to a board you haven't even used ?

What firmware does it have at the moment ?
For me marlin is the worst out there. Repetier, sailfish, smoothieware - all hands down better. Easier to use with better control panel options.

a 300x300 should be fine.
does it not have a heated bed already ?