I just bought rsr super down lowering springs for my Lexus ls430 and was wondering do I have to replace the strut by chance? If so which brand is recommended? Thank you

 

Not unless the struts are leaking/worn. I assume you don't have the air suspension, or else you wouldn't have purchased lowering springs. You'll probably want to replace the strut boot (accordion looking covering for the strut rod) and the bump stop. My car's struts were original at 20 years old, and the bump stops were completely deteriorated. The strut mounts are usually sagging as well, and should be considered for replacement.

Edit: Given that you are lowering the car and will have them apart anyway, new struts may be a good idea, regardless. Can't recommend a brand with damping that will match your spring rate. The Sport struts may be a good option given that they are valves for stiffer springs, but I have no experience to draw from on the subject.