Hi and welcome to the forum OP.
I see you plan to try a few, but personally I'd always go for a 6 cylinder one (even my BMW Indy who is ex-Sytner recommends avoiding the 4 cylinder models) and in any case a straight 6 is one of the Z4s unique features!
There was a facelift in early 2006, and the engines changed.
Pre-facelift 6s used the M54 engine in 2.2 litre (170bhp), 2.5 (192bhp) and 3.0 (231bhp) sizes. But as they are all fundamentally the same engine, running costs and economy are all pretty much the same.
The facelift 6s used the N52 engine in 2.5i (177bhp), 2.5Si (218bhp) and 3.0Si (265bhp) formats - again running costs are pretty much the same for all three. The N52 is more sporting, but does come with a couple of foibles. One is it has no dipstick (you check the oil level with a button on the dash via a sensor) and it has an electric water pump which costs at least £250 for just the replacement part. But if you find one that has already had a new one you shouldn't have to worry about it for some years.
Roof motors can be a problem as they are located under the hood drains, but there are a few members on here who can revive tired ones and relocate them out of harm's way in the boot for a sensible price.
Of the 3 you have posted links to I'd pass on the 2nd one as it's a 2.0 litre 4 cylinder one! The third one looks like the best one, but it is a 2.5i rather than a 2.5Si - you might be better off looking for a pre-facelift 2.5i or 3.0i. That one does have rear parking sensors, but you can't see which seats it has!
When you mention Sport seats be careful with descriptions - the standard seats were called "Sport" seats by BMW, but what you really want were called "M-Sport" seats. And to add to the confusion on facelift models "M-Sport" seats were standard on Sport models - SE models got the "Sport" seats as standard, although some do have the "M-Sport" ones as they were an option. If you really want the M-Sport ones make sure you find one with them - even used ones sell for upwards of £800 a pair!
Other desirable options on an E85 are heated seats and bi-Xenon headlights (the standard halogens are pretty poor). If you want it cruise control is a fairly easy retro-fit, and you're about right on hard-top prices.
The websites that used to give a VIN from a registration number don't seem to work any more, so you'll just need to ask the seller for it or get it from the visible VIN in the windscreen.
I've added a photo of the M-Sport seats, which I think is what you are looking for!
Anyway good luck with the search, and don't forget the photos once you find one!
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