I don't think a bandsaw would be my first choice for cutting styrene sheet. I assume you are talking about normal modelling thicknesses, 1 or 2 mm or less. A sharp blade will do a neater job. That said, if you have a large number of cuts to make in big sheets of material I can see the attraction. You really want a blade with lots of tpi, perhaps a metal cutting blade. (I should add that I have never tried this and it might be that the soft plastic clogs the teeth too readily. Some experimentation is probably needed, keep the speed as low as your machine allows.). If you use a conventional wood blade, especially a skip tooth blade, you will tear the material rather than cutting neatly. The same applies to thin plywood. You might also want to be extra efficient in removing the dust/scraps of plastic as if they get into the moving parts they could melt and gum up the works.
In terms of blade width, it really depends how tight a curve you want to cut. Narrow blades cut tight curves but aren't as good on straight lines. Wider blades, vice versa. If you are doing mostly straight lines or gentle curves (radius 6" or more) then a 1/2" blade will probably be best. If you plan on lots of intricate curves, 1/4" or less will fit the bill.
There is an almost infinite amount of advice on YouTube and Google about choosing and setting up bandsaws. Have a read of some of that before spending your $$.
Greg