A good sized garden can be maintained with almost any descent tractor (large lawn tractor on the small end of the scale). Post hole digging means a large heavy tractor where I grew up with tough soil. For brush clearing, too small a tractor will make this a tedious task at best.
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I don't know how you guys use a PTO powered post hole digger, but it doesn't take much tractor to power one. Large heavy tractors don't give you any benefit more than small ones. They just turn the auger, no down pressure on 3 PH ones. Hydraulic powered ones that mount on FEL would be nice for very hard soils but they are going to really cost you. Most people now use T post for fencing except for corner /anchor post which you need larger wooden post for. YOU do have to be careful not to let it auger into the ground, lift it up to clear the hole several times and especially in soft clay soil hold it from digging too fast or if will auger in and no tractor can pull it out. I have used them on as small as a 750 JD (19 HP) without a problem. You likely cant use them on the SCUT's like the Kubota BX models due to lift height, put power wise, you can get hand held ones with 5 HP engine on them that work fine, they just work the dickens out of the operators (most take 2 people to use.
As for a tractor within your budget the small Fords might be your best bet. There are plenty of them around and most have good parts supplies. I would look for something with live PTO or at least a two stage clutch especially if I would be using a rotary cutter (bush hog) 861 Fords or the 2000,3000 series Fords can be had within your budget. I would look for a diesel engine if possible as the fuel stores better than gasoline. Even with fuel additives like STABIL, gasoline starts to sour after about 1 month and with all the alcohol in it, it attracts water like crazy.