The SMALLEST tractor I would buy would be a 24hp small frame CUT. For Kubota that would be either a B2410 (their deluxe model) or the B7610 (their economy version). Given that you want to stay with a fairly small tractor, the size I would prefer, is their mid-frame unit with 30hp. They have 2 good choices, the B2910 (deluxe version) or the lower priced/lower featured version, the B7800.
I would very quickly gravitate to the 30hp units simply because you are starting out with 10 and looking to end up with 20. 10 acres is a nice size property to have to maintain, 20 is big to maintain with a small tractor. And even a 30hp tractor is not too much for 20 acres, I'm sure you'd find a lot of people who would suggest going up to the 40hp range.
You said you want "useful and cost effective" . . . seriously consider moving up in size. I think you would seriously regret a BX very quickly, and probably regret a 24hp tractor the moment you get that extra 10 acres, but you might regret it long before that.
I think what you really need to do is lay out your tasks, then rank them by importance.
If you are going to mow about 25% of that property (5 acres of the 20 total) then you want a MINIMUM of a 60" finish mower, that will require a 24hp tractor. But a 72" mower might shorten that task considerably, which would have you go up to a 30hp tractor.
You want to build landscaping, and you say you have rocks, so you NEED a front end loader. Consider how little the BX or the 302 loaders will handle and that will quickly move you to the 352 loader on the 24hp tractors, and probably will push you up to the 402 loader on the 30hp tractors.
One of the nicest things about the B2910/B7800 Kubotas is that they are small 30 hp tractors. They fit in the woods much better than larger frame 30 hp tractors, and since you say you have woods I thought I would point that out. I use Kubota & New Holland tractors and while I prefer to use a NH TC24 in the woods over the B2910, I will readily admit that the B2910 is still a small machine and will fit into places that the larger 30hp machines simply will not go. The other nice thing about the B2910 is that it is a light weight machine which makes it great for mowing if you have clay soils as it will leave far fewer ruts in your yard. If you have sandy soils you can operate a heavier tractor without rutting, but on clay light tractors are a real advantage. At 4 years old, the B2910 has never been in the shop, so from the value standpoint, I certainly can't complain! A B7800 is less expensive, but has fewer features, you can decide for yourself if those features are worthy of consideration for your uses.
By the way, please fill out some of your profile!!! If we know roughly where you are, what climate conditions you have, what your soil conditions are like, etc. then it will be much easier for people to provide you with more specific recommendations, and in some cases you might even find someone close to you that can recommend a dealer, or even let you try out a tractor.
