riken made great,bulletproof hypalon boats so the choice to buy it or not would be more on it's condition. 20 year old hypalon can be nearly flawless if reasonably taken care of.... or shot.... or someplace in between. just depends on the life and care it had. give it the soapy water test, look at the seams,d-rings,test the valves, patches,ect. if it's online then grill them on every detail,get lots of photos and don't take any shit or lay down if it's not as described when it shows. and make that known and clear before the sale to help keep them honest. no need to be a dick, just firm. if they don't answer your questions to your liking or offer the photos you want, get a little vague in language on how long it holds air then walk away. not worth the risk of getting someone's problem boat.
military valves are fine and durable. the weakness in the design is not to over-tighten them as that can break them. just snug is fine.
diminishing tubes are cool. good for extra room inside for gear,ect. not something to worry about, i'd call it a plus really.
my first raft was a riken,btw. good stuff, no doubt.