The major issue I see with your plan is borrowing the neighbor's tiller. I hate borrowing or loaning stuff out. When you're using his tiller, and you hit the piece of rebar, buried concrete block, or whatever, and break something, he's stuck without his tiller, and you've broken his tool. Can you afford to replace it for him, or take time off to fix it correctly, or get a service shop to do it?
If you're tilling your garden, it's time for him to till his, too. As the loaner, he's in a bind, because he doesn't want to be a jerk, but it's not right for him to do without or have to pay for damages, either. I'm not wanting to preach or impose my views on anybody else, but borrowing things like that isn't worth it to me.
A 20x40 foot garden is no problem for your machine, pulling a 5 foot tiller. These tractors were designed to till, and have very low gearing as a result. Even if it's a heavy US built tiller, in 1st gear, low range and 540 PTO rpm, you'll be fine as long as it's not hard, unbroken soil. If it is, I'd figure out a way to break it up some. Rippers on a box blade work well, or what some people call a "middle buster" or potato plow. Take more passes, who cares? If the blades are good, any of the tillers I've used work the soil deeper than necessary for home gardening anyway, so you don't need to be able to power through soil with the box fully buried.
I'm not trying to start a flame war at all, but transit's remark about not planning sufficiently for your needs doesn't make sense. You don't need a 30 to 40 hp machine to prepare an 800 square foot garden. Your purposes, as I recall, are mostly slashing and driveway tending. Your machine will do fine for that, and kept a lot more money in your pockets, as well as giving you finer control and tighter access into any areas that you may not be able to get a larger machine. And there will always be times that you'll wish you had something bigger. There will also be times you wish you had something smaller.
My advice is decide if you want to borrow implements. If yes, go try it out and use it. Things will work fine, there's a learning curve no matter what. Take pictures so we can see things, then post them.
