I've considered Boxsters and F Type's but realistically I'd have to spend £30k. I'm not sure I'd suit a 6 series, but have considered Z4 35i/is at great length.
Current owner of a Z4 35is here, abit of info below:
Firstly the Z4 35is is more of a GT cruiser with fantastic straight line speed, rather than a "sports car" per say like a Porsche Boxster for example. It has the N54 engine out of the 1M, and the DCT out of the E92 M3, they can make circa 440bhp with a couple of basic bolt-ons and a tune (Downpipes, Intercooler, Tune) and the aftermarket community for the N54 is huge. The car can be tuned for as little as £300 with an android application that has some fantastic OTS maps done by wedge performance in germany. Oh and the exhaust note is intoxicating, lovely burbles on over run. Once tuned the car rips pretty hard, and if down the line you want more than 440bhp there are lots of tuning options for the N54.
The folding hard top is really nice, and makes it a great car to drive with the roof up or down, with lots of power when you want it, and looks fantastic too. Fuel economy is pretty bad, depending on how you drive it, could probably get around 30mpg on the motorway driving carefully, regular driving expect circa 15 - 20mpg.
They are great cars but they do have a few niggles, some of which are common for the N54, I have listed the common ones below:
Injectors -> Old injector versions prone to failure, circa £700 for a set of 6 if you shop around. Parts only
High Pressure Fuel Pump -> Seems to be a common failure on the N54, circa £700 - £900. Parts only
Valve Cover -> Plastic POS that cracks and leaks oil, circa £400, parts only
Turbos / Wastegates -> Wastegate rattle is common, turbos are generally pretty solid. Not to sure on prices on these
Adaptive Shocks -> Prone to leaking / failure, circa £500 a corner, parts only
DCT Sump Leaks -> Another common fault, not sure on price once again
Other than that they are pretty much solid, a BMW warranty can be obtained if the above worries you. I believe it is circa £500 per annum fully comprehensive provided the car has less than 60k miles and has full service history from VAT registered garages with official BMW parts used (with invoices as evidence).
I would avoid buying from a dealer as the mark ups they seem to put on the 35is are huge, I have seen 2014 cars up for sale privately at £18k, to then appear at a dealer at £24k a few weeks later. There are not many around, so you may have to look around for a little while due to rarity, but my oh my are they a great car, beautiful interior too.
If you have any questions feel free to ask, happy to take you out for a spin / give you an in depth tour of the car if you are near by!