Here is my advice: Unless you already know a lot about the V8 conversions, I would stay away from them unless you really, really trust the seller. If it's from a reputable person in the Z community that is known for quality work, then yea, go for it. The problem is, it is really easy to get the wool pulled over your eyes when buying such a modified vehicle, because you don't necessarily know what to look for unless you have already done it yourself, or you have researched it so much that you know the ins and outs of a V8 conversion. Not to say you shouldn't buy one, you just have to be careful. If you buy one, it could end up not even being close to the setup you want or really need, or worse, it could be just plain unsafe because of a poor install or corners cut on things like chassis reinforcement, etc.
There is a thread that has been going on with people talking about how much they have spent on their V8 conversion, it might be a good idea to go through that thread just to get an idea of what a real conversion costs, as well as look through some of the build threads to see what goes into it, so you might know better what to look for.