Whichever you decide, try to remember your original premise of 'cheap roadster'.

I've just gone through a similar process, only I was planing on spending maybe £3.5-4k on a tidy BMW Z3 2.8.
I couldn't find what I wanted and ended up spending nearly 2k more than I'd planned.

I set my heart on a blue one and when a mint Blue 3.0 turned up I went to have a look, just to see what the dearer 3.0 was like, with no real intention of buying it...The other 'compromise' options were considered long and hard, but in the end I thought that I'd only regret buying a colour I didn't like with the smaller engine, just so I could have a convertable (never owned one before). I'm picking mine up on Saturday and can't wait, even though I'm lucky enough to have a brand new 320d Touring for work the idea of an 11 year old convertable is more appealing.
I hope you manage to find a good example of whichever car you decide on. Ask lots of questions before travelling to view as many seller's idea of 'mint' isn't necessarily what yours is.