We actually have a Z3 2.2i so I feel able to comment & also clarify.
I agree that's Z4 3.0i money and the Z4 will be a better drive, is more rigid & better dynamics........That said clean unmolested Z3's have been levelling price wise and in some cases rising. IMO the Z3 whilst more dated has nicer & prettier lines & a modern classic look.
That said I have run in small group Z4 convoys with 3.0i examples driven progressively & generally kept with lead pack........albeit my concentration was needed & mine was worked harder. In terms of driver involvement I certainly feel what is going on when pressing on.......some will like that others may be concerned at the thought of having to entertain this.
Contrary to what has been said the Z3 2.2i is a Straight 6.....only the 1.8 & 1.9 are 4 cylinder cars.
IMO the 2.2i is actually quite a nice engine. It had a modest quoted 170 bhp when new. It is not a rocket ship. Official 0-60 is circa 7.7 secs and quoted top speed 140mph is arguably ample. Some might say that even with top up the wind noise can get a bit lively approaching 130mph
In healthy condition the 2.2i is responsive and standard exhaust has a nice sound with small pops heard on lift off. I don't subscribe to premium fuel for sake of it but on this car I believe can sense the benefit. I have driven 2.8i and that engine also used in 7 series of the era - capable & more low down albeit arguably a little less involving than the 2.2i depending on how driven.
When mentioning the Z4 by comparison I mentioned rigidity. The Z3 is IMO a fun car to drive however there is some flex. The 2.2i IMO is thus well suited & proportionate to the chassis.
Some have made negative comments re the chassis of the Z3. I find this odd as the rear is based on E30 as in the much loved E30 M3. The front end is E36 based and plenty of happy E36 owners out there. Indeed the E36 compact models have a quiet fan base with many liking the communicative handling of the 318ti.
I viewed a few Z3's and drove a fair few too. As with other models there can be differences between cars & quality not always associated with odometer readings.
Personally i think that car is a bit O.T.T. price wise - if your friend wanted to get out after the summer, ( as many do), I suspect he might struggle to see his money back. I would want proof it's mileage is genuine and would be a little concerned in case the car being stood for long periods might mean there are cobweb related issues. Sometimes if cars are left stood they can protest !
I have seen 2.2i cars with 40-50k for under 5k. I recently saw a mint low mileage 3.0i for £6k on a forecourt.
I think he would have to love Z3's to spend that much. If looking to spend big money on a Z3 then I would be tempted to hold out for an S54 engined (more reliable), Z3M. They come up occasionally for the right money and IMO are better value than the much desired Z3M Coupe (sometimes referred to as the bread van).
If I wanted a Z4 with potential for some residual busting rarity then the Alpina Z4 S with Lux pack would be on my list........It's rare to see Alpina models with a proper gearbox too.
Of course it all depends what he wants. If serious re driving dynamics then arguably an S2000 might give more smiles per mile. NC shaped MX-5 may seem a boring choice to some but with RX-8 derived underpinnings they go around bends nicely.
Porsche Boxster S 987 & R171 shape SLK more money but modern dynamics and looks for under £10k.
I've typed far too much as is often the case when discussing buying/selling & used cars.
HTH
