My impression is that the ATN Topclimber is an adaptation of the frog ascending system. The frog ascending system is used for vertical work and is very popular with cavers. It allows unassisted ascent and descent, but does require some technique and a complete system probably costs more than you're looking at (but less than an ATN Topclimber).

Gonzo Guano Gear has a complete system for $250. Karst Sports has the system (but not including the seat harness) for $215. I also carry a separate static line for climbing, which adds more $$. And if you want to add a descender to rapell down the line, that's even more $$.

And if you are using any climbing harness, be aware of harness hanging syndrome (HHS). Hanging immobile in a harness for even a short time (just a few minutes) can cause loss of conciousness and even death. The experts still don't know exactly what causes HHS but they suspect the leg loops restrict blood flow to the torso. The way to prevent HHS seems to be to keep some body parts moving. "Immobile" seems to be the key word.