The higher the purity, the lower the contaminants, and the better for your assay. Look for purity assessment by multiple methods such as SDS-PAGE, endotoxin levels (LAL assay), mass spec, and analytical reverse-phase chromatography.
Lyophilized proteins are usually more stable than proteins in suspension, look for proteins shipped in lyophilized forms, but check appropriate stability and recovery checks have been performed.
Animal-free and animal-derived material free are not standardised terms, when buying animal-free proteins make sure you check the manufactures policy for what animal-free really means.
If you require extra information or help will you be able to quickly get in touch with a growth factor expert? Are any guarantees offered?
If you are performing a large study and want to ensure lot consistency look for suppliers who support stock reservation, bulk ordering and custom vialing.