I teach auto shop in a high school.
buy the E30. They are mostly built proof. Lots of fun to drive. + they use the same parts network as your current BMW (never a bad thing to have a good relationship with a good parts guy).
I hope you have a knack for electrical trouble shooting (or at least a will to learn) as BMW's of this era were full of electrical gremlins that pop up as the cars age.
Buy the Bentley Manual: its well worth the money! There are NO better manuals out there.
Remember: German cars are designed by obsessive engineers: you will be amazed at some of the solutions they came up with while working on your new toy.
There is no better way to learn than by getting your hands dirty and figuring things out while you follow the detailed procedures in the Bentley.