I can't comment on the 1.8 as I've never had one. On this community we will be biased and tell you how good the 75 is. We all love them.
The drive on all of them is great, they are great for cruising. From what I understand, the non turbo 1.8 isn't going to light any fires though, but maybe that will help keep the insurance down. And that is probably something you should research first, what will the insurers want to sting you for? The 75s are big, and not cheap to repair, so the insurance may be steep. You need to check.
The other thing I need to say, is be careful what you buy. There's lots of people who have bought badly looked after ones of these, and although generally way better than you might be told, if you get a bad one, it could cost you. Remember these cars were in the region of Â£30k new for higher spec ones, so they are great, but plenty of expensive things to go wrong if you aren't careful.
Many of the answers you are after are tucked away on here. Changing the stereo for example, depends on what you want to do. There are a variety of OEM solutions, including pretty high spec satnav systems from 75s/ZTs, or even options from BMWs. There's also more up to date Chinese systems that some buy that have TV, satnav and so on. It all depends on your taste and the size of your wallet!
It may be worth meeting up with some owners. I'm not sure where you live, but there are local meets all over the place, and in the Midlands we have Nanomeets where we spend weekends helping each other to upgrade and maintain each others cars. As a potential owner you could learn a lot from this kind of thing!