No seat and no gears FTW.
If you are talking natural I would say go for something with a longish wheelbase (1070+) and no seat. Seats and gears is really up to the individual, and how serious you are about the riding you do. If all you would ever to do with the bike is ride trials, you would find you would only ever use the one gear anyway and are just carrying useless weight around. Just dont think about commuting with it. Riding more than a k standing, and with 18-16 ratio is so crap!
If you are even thinking you could handle doing a whole ride standing up then go seatless, you wont regret it. Most of the seated frames are shorter in wheelbase and more so aimed at street riders but there are still a lot of people that ride seated trials bikes very very well no matter what the terrain.
Echo range of frames are good to get started on. Cheap and tough. Or look on Observedtrials.net, always second hand bargains popping up there.
- Jon