always buy a complete bike where possible the costs associated with converting a std bike might seem easy 1 piece at a time but add it all up and your on a hiding for nothing.
essentials when buying a trials cub.. pvl ( electronic ignition) wide swinging arm alloy rims (21 and 18 inch) minimum std heavy duty cub forks cub hubs front and rear
anything else is bling / nice or simply un necessary but look for domino levers, renthal bars, tank without a sealer, modified, narrow rear subframe, alloy oil tank, alloy mudguards, folding kickstart. understand what gear ratios/ size sprokets it has, branded trials shocks, decent tyres with a square edge if worn.
loads of cub stuff about expect to pay min 2k for a running useable bike expect to spend another 2-500 making it your own.