Using a 'new' car as a rally car means you have to buy one still. And I'd rather find $1000 civic shell on craigslist than have to shell out $15k on a Sonic.
The only truly important thing is it's what you want. Anything can be made into a rally car. If you want to win, well there are some that are better than others. If you want it to be cheap, there is another short list of those. If you want it to be competitive AND cheap, well then you are probably just going to build another subaru or vw like everyone else.
For more silly inspiration: