Personally a good bar and bumper plates (training bumpers are much less expensive then they were thanks to Crossfit) and rack (or half rack) are the first things I'd buy (well they were the first things UI bought). Yeah they can be expensive but they will last you a heckuva long time.
Are the Olympic lifts replaceable? No. If you do them properly they provide a unique stimulus that you don't find elsewhere. However if someone cannot do them properly I prefer that they drop them from their program.