Those two speeds are good engines, the clutches suffer a bit since they are made of rubber. You can run a 70 kit but have to really be aware of how you’re riding to avoid burning out the clutches. You want to make sure to run the right non-detergent oil. I think $450 is reasonable if it’s mint looking. The Basic’s have gas tanks that are part of the frame tubing, so make sure it looks alright inside. If it needs tires/cables and has rust around the edges, and the chrome on the wheels looks poor, it’s probably spent some time outside and I wouldn't pay a penny over $300.
Those two-speeds can rip good with just a pipe, good intake and jetting - though there aren't lots of options for new pipes right now, and I don’t know if the mlm sidebleed works well on the two-speeds.