Really interesting thread, from my point of view, cos I'm in a very, very similar predicament. The only real difference being that I'm not planning to purchase until next summer, when I turn 22.
My current top choice is an S1 Elise. For between 8-10k I'm looking at one of the early base models, but since they're the lowest insurance group, it's the most sensible choice anyway.
I'm not sure how much your dog would enjoy an Elise, but since you're looking at 7 replicas and MX5s, I see no reason why the Elise couldn't fit your bill.
There's certainly a question mark over reliability, but that's generally always head gasket failure, which can be avoided by driving the car correctly (basically not thrashing it until it's warmed up).
The other car that I looked at recently was a Mitsubishi Galant VR4. Not very similar to the Elise, but with 280bhp heading to all four wheels, it's gotta be fun. You can pick them up for around £5,000 or so, too. The only problem is that I believe very few of them were imported by Mitsubishi and those that were imported were all autos, so if you find a manual, it a grey import. Bummer.