Sound advice indeed. I'd say aim for 2 full squads of Harley troupes (20 models), 6 Mimes, And probably a Shadowseer model. My personal advice is to convert anything and everything you can, however when you look towards Death Jesters, I would say get the archived models, OR spend extra cash and get the Dark Reaper Exarch models and/or Phoenix Lord Harvester of Souls models. I still say that the actual 2nd ed DJ models are better.
Guardians could be convincingly converted to Harley troupers.. I'd say maybe Dark Eldar warriors for Mimes. Dark Reapers or 2nd ed models for Death Jesters. I have a personal conversion idea in mind for the Wraithlord, so I'll not give you ideas on that... hehehe. Venoms are great. I'd say have 1 in your 1000, and add another one by the time you reach 2500pts. Jetbikes are convincing no matter which (E/CWE/DE) bikes you use, providing the right paintjobs. Solitaire, I'd advise a nice converson on the Callidus assassin, or maybe some creativity with a plastic beastman (after all.. a solitaire is suppose to play the part of Slaanesh in The Dance).... Shadowseer and the Great Harley is more or less personal choice on what you'd like it to look like.. I think that's the flavor to the Harlequin army... convert to what you think things should resemble.... just given a few guidelines on things. It's probably why the dedication to a Harlequin army is recommended for the experienced player.
*Note* I do not discourage a n0000bie from starting with Harleys... just remember it will take alot of patience to create such a beautiful convincing army.