I owned a 2000 Ka for 3 years and it was the most reliable car I've ever had. They are a good runner, and are basic so parts are cheap etc. Only issues it ever had were needing new wheel bearings (twice) and unfortunately Ka's tend to have problems with rust on the body shell which you should look out for.
Saying that, I now drive an SRI 2001 (C shape) Corsa and it is a million steps up from the Ka in terms of smoothness of the ride, quality of the interior etc. The Ka is really basic and it does show but I did love mine at the time. It has given me less mechanical hassle than my Corsa overall.
But, 4K is a lot of money for a 'good A to B car', and at that price it will depreciate quickly. If you want a Ka, you can get a perfectly good one for 500-1000 quid. I wouldn't worry too much about mileage on the clock on the Ka as they are really reliable cars. I would be concerned about things such as age (low miles and big age could mean it was a town run-around so lots of wear on clutch etc.), and rust-- as the Kas 'body' often goes before the mechanics do.
If you have 4K to spend, you can certainly afford a higher end car though.