For modelling purposes you would do well to use good quality brushes designed for use with acrylics. My favourites are from the Daler Rowney System 3 range, but that is purely a personal choice - there are many other ranges that might suit you better. The type of bristles will be determined by the size of your wallet. If you choose sable then your cleaning regime will need to be extremely rigorous. Never leave any residue on the brush at all. Ever. Synthetic bristles are more forgiving, and will stand up to quite harsh treatment. Mixtures are, as you might guess, somewhere in the middle. You don't mention whether these brushes are to be used for general painting, weathering or both, but you can cascade them to avoid unnecessary expenditure. Start their lives off with general painting and then as they get worn, cascade them to weathering fluids and finally weathering pigments.
If you have a local artists' supplier or art shop then a conversation with them would help you in your task.