Hi Bully Jock welcome to the Forum.
Quite a few forumites have bikes or came from bikes (me included) so you've come to the right place.
£7K will put you in the market for any of the first versions of the Z4 including some 'facelift' versions (post 2006). The 'new' Zed came out 2009.
The 2.0 litre is a good choice for running costs and gives you the handling with fun performance.
The 6 cylinder 2.2, 2.5 and 3.0 litre sixes are well thought of and liked for their engines - smooth and powerful (2.2 needs revving)
The 3.0 litre is the most popular and can offer reasonable mpg, running costs but with excellent performance
The pre facelift 3.0 litre is 231bhp, the post 2006 3 litre is 265bhp; either gives strong performance that you will enjoy if you're used to the performance of a bike.
There are Sport versions that offer sport seats, steering wheel, suspension and wheels; SE versions are 'softer' and offer a better ride.
Most owners change the Runflat tyres for non runflats for better ride (and replacement costs often less).
Many owners fit upgrades - sport seats, different wheel designs and there are many options available which add to the practicalities and desirability of a Zed - Satnav, uprated stereo, parking sensors, cruise control.
As the Zed is a very capable sports car a good service record is recommended and a well cared for Zed should give many miles of enjoyment upto and past 100K miles.
Roof motors can stop working and are expensive to have fixed/replaced and over time wear means the replacement of joints and bushes are needed to freshen up your Zed - but by using a specialist rather than a dealership, owning a Zed can be very reasonable in terms of cost.
Take your time and read through the sections on the forum to get to know the Zed better. Look out for Zed's in the for sale section as a well known and cared for Zed can come up at a fair price. Decide what Zed you want and then be patient - there are many out there (not all good mainly as they will need money spent on them) and although you may have to compromise on colour or options you will end up with an exciting and characterful sports car which will thrill you and give lots of smiles per mile!
Lots of knowledge on the forum so just ask away and good luck with your search.