Fixies are pretty easy to build from scratch, just budget the time to do it properly. If there's a bike co-op or collective near where you live, that would be best, as there are some specialized tools you need to do it right.

When you say "from scratch", are you converting a road bike or building one up from the frame out?

If you're building it from the frame out, make sure you've allocated the necessary cash, as it adds up fast.

In terms of the tool list, you may want spoke wrenches (and a truing stand if possible) to make sure your wheels are straight.