An
M5040 (or the JD 5203 you were also looking at) would have the size and power to run slightly larger implements, specifically a 7' or 8' rotary cutter, and a 6.5' or 7' box blade. I have a 6' rotary cutter on my JD 5105 (47 PTO hp, a tiny bit more than an
M5040) and have double the power I need. If you are mowing large areas, I would seriously consider a larger cutter. However, if you are mowing tight areas, or extremely dense vegetation, the 6' may be the way to go.
As for a box blade, it must be as wide or wider than the rear track of your tractor, or it won't work well for multiple passes. I think the standard track on an
M5040 is around 72", so you would be barely OK. You should have the power and traction to use a wider box in most conditions, and I would consider going wider.
From my own shopping a year ago I seem to remember the Kubota manual recommending a box blade no wider than 6', but I think that is bad advice. Supposedly Kubota is ultraconservative on such ratings.