We use a New Holland TC30 for our property. It's 3 cyl. , 30hp diesel, 25 PTO HP (I think), and has a 5' bucket on the loader. It has the R4 industrial tires, which I find to be a very good compromise between the ag. and turf tires. They have almost the float of turfs, but decent traction, except on our rather steep driveway in the winter. Chains help there. We plow the driveway with a 7' rear blade (going backwards is kind of annoying, but you get used to it), and we have a 6' JD light duty brush hog.
My only complaints are that the transmission needs to be synchro-shift (I think larger NH's are), and the engine takes forever to warm up when the air temp. is under about 20*. When it's this cold, the loader doesn't cycle too fast either. If you are planning on rototilling the garden, I wouldn't get too large a tractor, considering anything larger than the tiller width (about 5' typically) will make it hard (or impossable) to till the middle of the garden with plants on either side. As for the post-hole, tractor size doesn't matter too much, just choose wisely on the actual size of the digger frame and the size augers it will take. I can't explain much about the trenching deal, but I'd personally leave that to someone with a Ditch-Witch, or you can get a back-hoe attachment, but those are difficult because you only get so far before moving the tractor, and they are rather expensive ($5k-$8k depending on size). It also takes a while to remove them, so it's not a practicle thing, unless you are a landscaper who will use it all the time.
Our tractor, with loader and "Woods 72" blade came to about $15,000 brand spankin' new.
Sorry to be long winded, but a tractor purchase is something that needs to be considered like a car purchacse, look at all your options and talk to lots of people before making any final decisions.
I hope this helps you,
Good luck in your search,
Scott