I've got a pair of MBZ original sealed batteries from some newer MB that came with my car. They don't leak anything.
I'd recommend painting the tray (when you are done fixing the rust) with something like POR-15 that is supposed to be acid-proof, and add an absorbent pad just for good measure.
And seriously think about getting a welder and learning how to weld, especially if old cars are something you plan on continuing with. Its really a lot easier than you think it is. Just get a book, Haynes has a good welding manual, actually, a used small MIG, like a Millermatic 135 or a Hobart Handler 125, a small rental tank, a basic autodimming helmet (50$) and get practicing. You can have your own cottage welding business in no time.
peace,
sam