Buying any Porsche at the lower end of the price bracket but top end of your own budget is a stupid idea. I did lots and lots of reading, asked my local indie (DeutscheTek German car specialists) and have a friend who is an absolute Porsche nut having owned 8 of them in the last 5 years. The general consensus is that you probably need to spend between £2-3K in the first year fixing, putting right and doing preventative maintenance for peace of mind. The forums are full of people that will give you this advice, I had a friend that bought an ageing 986 and he spent £2k in the first 2 months. If your happy with that and can afford it then go for it. Just go in with your eyes open and don't be one of the idiots that goes "wow I can buy and run a Porsche for the same price as a family hatch".
If it was me. I'd spend the money else where. At the low end of the "fast car" bracket £15k would get you a Mk6 Golf GTi, not as fast but always fun, hold their value well, easy to tune with fantastic gains from simple remap and exhaust mods and they are ridiculously practical. 135i coupe, sounds awesome, faster than a Gayman and a relatively cheap remap turns them into absolute beasts. Again, it's also way more practical. Or, the real outsider. BMW do 0% on a 50/50 on the new M135i (so 50% of price up front then pay the remaining, hand back or trade with the equity after 3 years) funnily enough a little over £15k equates to 50% of the price. The car is absolutely awesome, blows the doors of any ageing cooking version Porsche, was a finalist in this years ECOTY, has been raved about in every review etc etc.
Just a point on the practicality. I just don't think there is any way you can ever describe any 2 seater car as practical. Yes, the gayman has a boot at both ends but it still doesn't make it practical. It's not great for a roof rack either, the roof is so short and when you have something on the roof you can't open the rear boot. I surf, my longest board is just over 7ft and have discovered this issue on a few cars. I would imagine a gayman with a 4m boat on the roof will be hideous to drive in the slightest wind. In terms of the boot in the Z4 coupe. It's the same as the roadster only it has the option of a bit more height. it's not a bad sized boot.