1938 323's are very rare, best you look after it well. ?
Yes, you can clean the existing hoses no problems. Any detergent will be fine, just make sure you flush them clean and let them dry really well.
Silicon hoses can be found here: https://roosemotorsport.co.uk/silicone-hose-kits/bmw-silicone-hose-kits/e30-hose-kits.html
And also other places, I'm sure Samco will do them but if you can afford them you can't go wrong with the Roose ones, they're exceptional quality. Having said that, as you've noticed the stock rubber ones are pretty durable, given the cost of silicon for really, very little benefit, I'd take a lot of convincing.