depends on your skillset
unless you guys are very handy with tools (and for you to be posting this...i dont think you are) i would just plonk down the cash and buy one. if you just go to work and get paid for the time youre about to spend on this thing, youll find that its not really worth doing
personally i like the idea of having an inground spa, and if i were to do it myself (maybe. depends) i would probably get a backhoe guy to dig the hole/trenches to my liking, put all the plumbing/air lines in, then use concrete that has those little bits of metal in it already, so that i dont have to reinforce with the usual 16mm steel rio bars bent to shape...then pebblecrete over the concrete for a nice surface. depending on the type of pump/filter/heater you get, this is a fairly cheap way to get a very nice final product. i never liked those fake plasticy/fibreglass ones.
gluck