IRO Mark V's aren't pre-built; Tony, the owner of IRO, will put whatever components you want on a Mark V. You can even do what I did and get the frame, fork, and drivetrain from IRO and piece together the rest of the components yourself. That way you get the peace of mind (i.e. warranty) of a new bike, while still retaining a bit of the "I assembled it myself, and it's unique" aspect.

I toyed with building a bike from scratch for a while, and also doing a conversion, but ultimately I needed a bike quick and this seemed to be the best, quickest solution (I ordered everything on a Saturday, and the IRO stuff arrived on Tuesday! The rest of the components didn't arrive until later...). Now that I have a nice, new bike and therefore no longer have an immediate need for a bike, I am looking into doing a conversion. I know a couple people that have done this; they first buy a nice new or used track bike and afterward buy a frame and do a conversion from scratch.

One word of caution, though (and I've said this elsewhere on the forums (sorry for the repeat)): I had Tony build me a wheelset, which arrived dangerously under-tensioned. I had to have it re-trued. Otherwise my experience with IRO has been excellent and I would definately recommend them.