like tvr as we all know something is bound to happen.
You don't know this, it's an unknown variable.
The Mrs has has an Alfa Romeo for the last two years, and guess what? It hasn't gone up in flames, or broken down once, contrary to popular opinion
Your choices are faaaaaar too safe for a weekend car. As biscuit says, the cars your looking at are great for doing 12-15k per year in. If your going to be doing a few K a year then get something special, or just keep the zed and buy an Akpro exhaust etc which you can sell on rather than loosing it on a Cayman/M3.
Don't forget on cars of this price, buying from a dealer your going to be paying about £2k in profit. Your £28k from a dealer is only actually worth about £26k. Once you start driving the car and adding the mileage, in 12 months you'll be getting trade bids of less that £20k, irrespective of the mileage. The rarity of 350z's/370z's have kept their residuals fairly strong but as Cayman's and M3's are ten a penny, your going to take a huge hit imo.