Few things I found when looking for a ride on mower - blade cut size, discharge location, grass collector size, transmission type...
I find the rear discharge for the cuttings is less prone to blockage than side discharge. For such a large garden, you should be looking ar a 42"cut, with a large collector. Ease of "dumping" grass also - if you can empty the collected grass without getting off the mower, all the better.
I've an automatic which works a treat (forward / reverse), haven't heard of issues with the manual gears though... Height adjustable cutting is a must... more levels to adjust to, the better is what I've found & also "ease" of changing the cut level.
What I would also recommend is for you to look at your grass area - can you get a mower into the corners, easy to maneuver around the garden. How often are
you planning on cutting the grass - we've had a lot of wet weather this "summer", wet underfoot in the grass, so heavy mower may leave indents in the garden,even though grass is "dry enough for cutting"...
Have you secure storage for such? Can you transport it if issues arise or if you aren't servicing it yourself... I find sit on mowers great if you miss a cut or two
during the growing season as it gets through it quicker / easier, but I'm back to the self propelling mower again as grass is quite low, and manageable...
If buying second hand, then make sure you get a demo of it, & it actually cutting. Most have sensors which you should test ...
BTW, I think the Jonsered one is a bit "overpriced" at €15K.