2004 should be galvanised and possibly nano paint, so rust should not be as much of an issue as on earlier 203's. Check for clunks from the front suspension, sign that ARB, bushes (centre bush is bonded to ARB so whole thing has to be replaced) and maybe rose joints need replacing. Also check service record for ATF/filter change - if it's not been done (MB claim the 203 auto transmission is sealed for life, but an ATF change is worthwhile, had mine doen at about 70k) it's probably worth doing.
Also check that engine will rev above 3k and kickdown works - if not, MAF is goosed and needs replacing - happened with mine within a month of getting it, luckily covered under warranty.
270 is a good engine - it was the journo's pick of the bunch when the 203 first came out - which is why I bought one! Plenty of grunt when needed, yet also pretty frugal, I'd get 45mpg on normal local runs and 50+ on a long run.