When the battery on my W123 280E gave up the ghost, I was quoted around NZ$250 for a replacement. Apparently, the W123 battery is a non-standard size, being a little lower in height than usual so that it fits under the hood.
So I went round to my local parts shop, and an older guy there had a mosey around under the hood, and then produced what I think is a Toyota size battery. It has the same other specs as the M-B battery.
It fitted in the battery tray and just squeezed under the hood with half an inch to spare. It doesn't need a drain pipe, and it cost NZ$70.