There is no such thing as a cheap rally car... you would need a roll cage welded in which isnt cheap to buy or fit really, fire extinguisher, decent suspension, decent rally tyres, and then engine wise nothing bolts straight in would all be expensive.
Best bet would be trying to get the standard 1.7 engine upto FRP spec, and maybe later on think about throttle bodies
I know were on a ford forum here but get a pug 205 gti great base for rallying,.. parts are cheap, there light and best handling hatch really ever.... you can pick up a 1.6 or a 1.9 for around 1.5k for a good un and cheaper if your great with a spanner...