Choose carefully - obviously a well cared for example where much of the possible costs are covered - tyres, brakes, suspension bushes, servicing just done (especially Inspection 2) will keep costs down - tyres are expensive if 18 in. staggered set (use non-runflats for a better ride and cheaper). There are surprisingly long servicing intervals. Driven steadily and as a commuter, the 3.0 will return 30mpg but use the throttle and it drops quickly. If you use the revs you pay for the performance (6 cylinders remember). Mine is a car used for enjoyment and seldom gets over 25mpg and often low 20's - but then like others, I'm an ex biker and it's driven like a motorbike. I would be searching for a 3.0si (post 2006 called 'facelifts' as there are a few subtle lights and body changes), available at you price point. The SE version has slightly higher suspension and a slightly less harsh ride. The Sport version comes with ///M sport steering wheel, ///M sports seats, ///M sport wheels and ///M sport suspension. Options can make a difference to the ownership experience - heated seats are worth having, park assist is reassuring, everyone seems to go for satnav though the system is criticised. Up-rated sound system is well received as well.You may have to compromise on these as the 3.0si is sought after and there's not as many about.
The Z4 has an amazing balance of performance and running cost - it really is surprising how they perform so well and just keep coming back for more on so little servicing. Like any other car, it can be expensive to put right if things do go wrong but by using a good independent you can keep costs down. Roof motors can be replaced for around £500 now.
Regarding RWD - it will feel different to what you're used to and can catch you out if you are enthusiastic and over confident but the Zed does have systems that will cut in and help (within reason) when/if you get to that point. Basically, RWD will likely result in oversteer and FWD understeer if you overdo things. Manufacturers think understeer is safer (also FWD is cheaper to produce as well as saves space). Like anything else it's a skill you will learn and improve over time - do a bit of research to find out more about this - it can be part of the ownership experience - the different feel of RWD cars are what attract their owner drivers - there's lots to learn if you're interested or you can just enjoy your Zed and not reach these limits. Just like any other car, you will quickly get used to the seating and vision in the Zed - watch out for kerbs, out of sight of the nose - it's easy to ground the front and kerb the wheels. As already mentioned they have terrific turning circles. One tip is to raise the seat until you are comfortable managing the Zed and sit closer to the steering wheel. Don't be intimidated by the performance - it can get you into trouble very quickly if you try and use all the performance all the time but with the sports button off there is a long throw to the throttle pedal and the 3.0 can be driven easily about town. I drive with the sports button always on as I prefer the faster action of the throttle but you have that option and it may help - do not be afraid to use the sport button though as it changes the nature of the Zed in a very positive way. The 3.0si has good effective brakes for the road unless you are really pushing them to track levels. The electric steering may feel a little odd at first - it can follow cambers and imperfections on the road and always feels lively - most of us use non runflats to help improve this and slightly soften the harsh ride.
You can get lots in the boot and with the roof down plenty in the passenger seat. Storage in the cabin can be a bit awkward but quite a bit can be stowed in the cubbyhole between the seats (and behind the seats in some versions) and the glove box. The door storage is very slim and limited. Everyone hopes for cup holders - they are at the ends of the dash if they are fitted and come out when pushed in - they do work but take care as they are not as well built as the rest of the Zed.
The Zed does look a far more expensive sports car than it costs now - it's always an event getting in the Zed and going out for a drive. I find it a very capable sports car that offers great performance but remains reliable and affordable.
Good luck with the search for your replacement