Hi,
I'm not a Juniper employee bu I've seen some statements. So Í'll try to answer your questions anyway.
Does the SSG-xxM series run the Junos as SRX new models? You can run Junos 9.5 on both so yes
Does Juniper Networks will discontinue the SSG-xxM models in near future? No, they'll support it for years comming.
What SRX model will replace the SSG-350M model? Hard to say. Look at the specs relevant for your situation (Interface requirements, throughput, number og new sessions etc) and look at the specs of the SRX's. On throughput I'd say a 240, but on other reasons maybe a 650.