More bother then it is worth.

To fit the muffler now, you would need some kind of adapter to fit the tiny stock pipe into the bigger inlet of the aftermarket muffler. This will be welded on. When the time comes to change the piping, the muffler will have to be cut off and rewelded to the new pipe.

I say, if you are concerned about price, buy a good muffler now. Save up for a while until you can afford to have the piping made. Once you have all your money together, go and have it done. This will save you labour costs now, and you will be much happier in the end with a custom cat-back with a muffler of your choice.