I've adapted old road bikes. One of mine now has a 3-speed IGH. Another is single speed with a coaster brake. So I'm familiar with these conversions, and I greatly enjoy riding them.

There are actually a few bikes out there with IGH's but it takes a bit more searching because they are still perceived as a niche item. If you want to adapt an existing bike, then there are some things to look for.

For instance, it will be easier to work with a steel frame with horizontal dropouts. The steel frame lets you bend it a bit to accommodate the varying standards for rear dropout spacing. Horizontal dropouts let you make up for the slightly variable length of the chain, which a derailleur would normally take care of. And naturally there are many of creative options as well. All of this is for the mechanically inclined, possibly not for the faint of heart. I've gone so far as to build my own wheels so I can have exactly what I want.

The drawback is that it's hard to come out ahead cost-wise compared to starting with an inexpensive mainstream bike. Derailleurs aren't all that ugly.