BulkBuy allows you to shift-click an item sold by a vendor and type in a quantity higher than the default one stack that the Blizzard UI will allow.

o purchase up to 1000 items at once with shift-click
o prompt for confirmation if more than one stack is purchased

It will not check for free bag space but it will show a total price in the confirmation dialog. I use an internal Blizzard dialog to do the prompting so it should be properly localized for all languages but I haven't tested non-english. The addon is 2 tiny files with no external dependencies so it should be very light-weight.