350z(top contender right now)
z4 3.0(a few locally that look decent enough for 5-6k - ideally need to test drive one)
s2000(will totally depend if anything decent pops up for my budget between now and me finding a good 350z/z4)
Biased post but basing off internet stats on the other two
The Z4 I would say is the better all rounder.
3L in a light body, I average mid 30MPG on my normal commute driving normally (Not that you should be looking at mpg in a sports car but nice to know) I tried an eco challenge
ONCE!!!! Cardiff to Bristol then back to Cwmbran, I was easily getting more then 40MPG (according to the trip computer)
Has a surprisingly big boot, I don't play golf but when my old man borrowed it he got 2 golf bags and still had room to put more stuff in.
I'm a big lad both vertical and horizontal and its pretty spacious inside.
Under 6s 0-60
The M54B30 engine as mentioned found in all sorts of cars such as the well known E46 is very reliable
Push past 4k RPM and it makes a glorious sound.
And its a great looking car.
And you can pick up a pretty decent low miler 3.0i for around 5k should be able to get an si for your money, Look around on the Z4 Forum tons of useful stuff and people always selling on there.
My gripe is the interior quality, besides from that I love mine.
If I had the money I would have went with the S2000, I bet its an exciting car especially with that 9k redline.
Also how much have Blobeye STIs gone up

Few years ago when I was looking at them they were like £6-8k, I'm seeing a load of normal spec UK cars going for £8k+ wow.