Well, obviously a 735i 5-speed would be VERY similar (from a cost of ownership standpoint) to your 535i. More weight means faster wear on some suspension parts, and there are a few more features to break, but it's a pretty similar car overall.
The 750's have a BUNCH of costly issues that can crop up, not just electrical, but a lot stemming from the massive complexity of that car and the relative scarcity of parts and therefore, the cost of them.