haha. yeah you just need something that has two sets of threads on the hub: cog threads (tighten to the right) and lockring threads (tighten to the left). You also need a cog and lockring. However, if your dad did the SS conversion, he might have already bought a wheelset that has a fixed side on it. You should show a picture of your rear wheel so we can make sure it's not already set for fixed use.

Even if you do need a new rear wheel, though, it will be less than 100. Cog and lockring will be like 25