My observations with mowers and toppers is that second hand they fetch a handsome sum often within a few hundred pounds of a new one. You have no idea how old they are and they can easily be ten years old, and until you spin them up to speed you don't know if they have hit a huge stone and are out of balance.
If I were you and had the money , sounds like you will use it plenty, I would bite the bullet and treat myself to a shinny new one. I'd recommend an established make like Fleming such that spares will always be available.
The one I bought is 5ft wide, which I have found to be ideal on the back of my MF 35 as it is just the same width as the tractor, so you can mow right up to edges , tight past gate posts and trees, knowing the mower will follow you nicely.
I looked for ages (and I mean ages) on ebay and dropped across one which I got cheaply...I didn't know the make and took a bit of a gamble really...when I researched it I found out they stopped making them 20 years ago....! So it's at least that old. It been fine so far but that was luck really, I would seriously consider new as I mentioned.
The mf35 is ideal for the job.