I've had good luck with Autozone's "Duralast" batteries. Their size 49-D fits the e32/e34 perfectly and comes with the vent tube & fittings. Be sure to connect the vent tube & make sure it goes thru the floor pan without getting pinched. To remove the battery, unscrew the clamp at the rear of the battery, slide the battery rearward as far as it will go, and then tilt it to clear the seat base. Newer batteries come with a plastic handle built into them and makes the job easier.
I know it's a pain to take the seat out to check the acid level but I prefer the old style batteries that have removable caps for topping off, I check it about once a year. I think the top of the line Duralast (and other brands?) are permanently sealed, but they still evaporate off the water & will die if they dry out.