This is a good point. The kicker is spending money on stuff that doesn't translate to a new frame if you eventually intend to migrate everything over. Watch out especially for non standard parts (parlez-vous frainçais?).

I love conversions, mostly for the utility of rack and fender mounts. But if you're leaning towards a conversion in order to save cash, source a complete bike where you can employ existing parts for most everything besides the rear wheel, cog, and lockring. And if you really want to stay cheap, there's nothing wrong with converting a threaded rear "suicide" hub if it's set up properly.