The best of that lot hands down would be the Mazda Miata if you want something with a sporting chance of being reliable and not turning into an endless money pit, with the Lexus coming in second. (Being a luxury model a Lexus has a lot more to go wrong with it as it ages.)

The rest of those vehicles will likely drain your wallet faster than they drain their gas tanks.

By the way, those picks don't seem all that old to me. One might consider my 1967 Rambler convertible to be old, and I know some folks who would even argue with that.

I daily a twenty year old Mercedes convertible the past two years, (2000 Benz SLK230) so I have some inputs to say on this. This is just your weekend toy so you have options depending on what your expectations are. I am familiar with some of these other vehicles.  

A Miata would be the cheapest to maintain if you can find one that has not been modded or abused. They're good, but super overpriced in this market. The Lexus SC is also hard to find at a good price, they're kinda rare, and since it is a Lexus they are getting marked up, but I still see some of them still running fine on the road. Both of these would be good, but the price will be high. Not sure if its worth paying such a markup for something very old already. Still should expect some maintenance on a old car.

The Boxster is an eye catcher, no matter what year, people will just see the Porsche badge. Surprisingly, these still haven't shot up in collectors value compared to the Miata. You can sometimes find a decent one for less than eight grand, but expect very expensive maintenance. It's a mid engine car so when it does need work, its not a simple as popping the hood. You gotta partially remove the convertible top just to see the engine.

I am not too familiar with Audis, but I do know they require the bumper to be removed to do many jobs on some models which adds to the time in the shop. 

I have family who has owned a CLK Mercedes for six years, not a convertible, but the coupe version. The drive train is fairly reliable and the engines during these years were one of the best in the modern era. Before 2006 the Mercedes cars were still much more simpler during this time. You will get some electronic issues, but its not that complicated. The CLKs were made fairly well since its basically based off an E class which are decent cars. They're taxis in Europe. Also working on the engine is not so bad, Mercedes lays things out fairly well in the engine so its not really a headache, but the maintenance rate is still more expensive than Toyota/Lexus so be aware of that.

One warning I will give about German convertibles is they tend to have automatic tops. They will deploy using a motor or some mechanism. While it is cool and flashy, they do go bad sometimes. On Mercedes especially, they use hydraulics to run the tops, the little rubber O ring in the hydraulic system goes bad and needs to be replaced. It is extremely expensive, if you ask someone to do it. The rubber seals last only like 10 years then needs replacing. I am not sure if BMW, Audi, or Porsche uses this system, but I do know they also use some kind of automatic top. I'd research any issues they could have with the top not going up and down. 

Can't really talk about Fiat, have no knowledge of that brand. 

You can tell I really like convertibles. You just gotta be careful not to get a money pit. 

Get a Honda S2000. Try not to get ripped off just because it is a pristine example with low mileage. Negotiate hard as their values are only going up. Otherwise, it is an excellent convertible.