If you are a BMWCCA card holder, you should be able to get at least a 10% discount from your local BMW Dealer when buying any parts or fluids (at least that's what I received when buying at my local Dealer).
Stay w/ the blue BMW coolant and the distilled water 50/50 mix is proper, but ifyou live in a warmer climate with no severe cold or winters, you can get away w/ a 70/30 mix (70 water - 30 coolant) - the higher percentage of water in the mix allows the coolant to be "cooler".
Also, step down to a cooler thermostat (might as well since you're going to be dealing w/ coolant anyway) - this is another good mod.
I even converted my 94 Cobra over to the BMW coolant - as after owning my prior 95 M3 and using the same coolant and seeing how it kept everything really clean internally, I decided to use the same in my Cobra.
I also purchase distilled water (PEAK sells such bottles and it's available in most parts stores).
Just flush the green stuff out of the system completely and once totally flushed, pour in your proper mix of blue.

Yes, BEHR is the OEM radiator for BMW's.
I think it was bimmerparts.com that used to sell radiator packages under a group buy (included water pump, water pump seal, thermostat, radiator and rad hoses) and the price was excellent - not sure if he's doing any such purchases now, but I guess it would not hurt to ask if a group buy could occur on just rads or a complete package deal...