The good old Massey 135 did all these jobs on farms of 100plus acres for years, they are about 40hp, these are quite sought after and collectable though. A 165 is about 60hp and possibly a bit cheaper, a bit more size and weight--- the baler and post knocker should be no problem for anything -- what mower do you have in mind ? The most common 5'6" drum mower is right on the limit for a 135--- more in terms of weight and handling than power -- especially if your land is hilly, the 165 however is well within its limits, power steering is a nice to have on these, but not essential. Also the 265 and 565, just newer models with essentially the same engine and gearbox.
Ive also used an international/McCormick 454, about the same power as a 135 but slightly bigger, not so collectable and immensely strong used it with a drum mower effortlessly.
Depending on your type of post driver you may need to adapt the hydraulics, some need a flow and return, and depends what tractors have had on in the past wether they will have the necessary connections and valves. Any tractor could be adapted- for a max of i guess about £150, just make sure you budget for it.