Would maybe look at vauxhall Astras and vectra, ford escorts and mondys, there's a seat ibiza on eBay with a years mot on it for £380, I'm sure with a bit of haggling and explain your reasons you'd get it for 333.
Would probaly look at your common cars as part etc are cheaper and easy to come by. Maybe worth looking at jap cars for reliability reasons as your planing on doing the rally in it