I've done this in the past many years ago. Bought a "poverty" spec car as it was at a good price (a Focus) then almost immediately wished I'd waited and spent more on the version I actually wanted. The car became a money pit as I started adding the missing bit's of spec. True I ended up with a car that looked almost undetectable from the one I wanted, but at a horrible cost. Coupled that with the fact that I could only find a few insurers who would cover the car meant I never did it again. When I came to sell it, I lost a shed load the money I'd spent on it. After all, if an LX spec car looks like a Ghia spec, it's still an LX and that's typically what it's worth come resale.
I agree with the others here, wait until you find the car with the spec you really want even if it means pushing the budget just a little further. You'll save money in the end.