Purification to obtain single compound from plant extracts is depends on the abundance of the metabolite and/or compound of interest to isolate by various methods such as crystallization, separation on column chromatography/ column purification of crude extract of the plant material. Semi-perparative HPLC and even Preparative TLC may also useful for separation/purification of single compound in mg level.
Generally a lot of polar to medium metabolites and all type of compounds will extracted into methanol, hence it may be the best for extraction, and more over depend on the nature of compound/metabolite the solvent for extraction is suggested.