OK, this is my speciality.
Firstly, you don't want a flail unless you are looking to mulch brambles etc with potential stumps in. A rotary will use a fraction of the horsepower. A single rotor topper will take some abuse well (I've cut three inch stuff with mine) and then keep on top of it OK when you lower the skid. It'll also work just as well in reverse as forwards, which is very handy for short work and corners particularly with a hydrostatic tractor where you don't need to stop and clutch to change direction. Or if you're looking for a fine finish then a three blade rotary with roller will make a lovely job.
You want, by the sounds of it, hydrostatic and 4WD. That means a modern compact, none of this Fergy crap. Kubota are lovely- but by far the best value for money secondhand are Iseki. For some reason they don't hold their money nearly as well as Kubota despite being every bit as good. I'd be looking for anything 26-35hp. For a slope the larger models are better as the wheelbase is wider. Whever you buy will want a set of front weights, but you can bodge something up for this or water ballast the front wheels.
As regards residuals- you just need to buy at the right money. Prices as mentioned are coming right down. I sold a lovely tidy 2014 TM3265 Hydro with just 800 hours for 6K. People have been asking 6k for a 1990's Kubota until recently. That Iseki won't drop much more than that till it's got 3000+hours on, and is to all intents exactly the same as a brand new Iseki or Kubota.
Don't even consider Solis, Siromer, Avenger et al. Nothing made in China or India is worth a wank. Keep it Jap. That means Iseki (2000s Massey compacts are Iseki underneath), Kubota, Shibuara (New Holland used to rebrand these). Parts for all those are easily available. I'd avoid Mitsubishi- never seem as good and no parts network.
How much are you looking to spend? If it's to be solely a mower then an outfront flail or rotary may work well. Usually Japanese (often Shibuara)and around 35hp. The ones that have hydraulic drive to the heads also have great flow rates so could be used to power a log splitter etc as well.