Given the skills of most mechanics here I’d try to find a low-mileage bike that has not had the bottom end rebuild. Look for wrench marks on the bolt heads that hold the crankcase halves together. There isn’t much compression, but the kickstart should provide some resistance and you should be able to feel each stroke through the lever. Any two-stroke is going to have a somewhat noisy engine when cold, and it’s going to make smoke too. Once warm however, most of the noise should go away, as should the smoke. Idle speed once warm should be around 1,000 to 1,200rpm and butter smooth. Not having valves, two-strokes rely on close piston clearances to keep the rings square in the bore to make cylinder pressure. Clattering or fluttering sounds from the cylinder / head area are a sign of wear here, and while the bike may run fast now, it will not run like that much longer. Two-strokes also need crankcase compression to pump fresh mixture into the cylinder, and they rely on oil seals on the crankshaft to do that. One of those seals is behind the alternator and is exposed to air. If that fails the bike will run weak, and not idle properly. The other seal is in the primary drive case and usually submerged in oil. When that one fails the engine will pull transmission oil in which burns smelly and white. If you run the engine with the transmission oil filler plug undone you’ll feel strong pressure pulses. Don’t do this in your favorite white shirt! Both of these engines have a sight glass for the transmission oil and if that seal is blown they will always be low on oil. A third weak spot in high mileage bikes is the reed valve. It’s just a simple one-way valve with petals to allow mixture into the crankcase but not back out through the carb. The petals can take a set when older and not seat properly. If you can get a look at the air filter you’ll see it’s dirty on both sides.
While idling with bike in neutral, pulling the clutch in should not make a huge difference in the amount of engine noise. If it does, the clutch hub or primary drive is probably shot. Once warmed up and with the choke off, throttle response should be instant with no bogging. In the first couple of gears the bike should pull hard from about 4,000rpm, almost as fast as you can twist the throttle open. Be sure the clutch disengagement is smooth as the clutch drums get damaged easily from abuse (being wheelied a lot) and they are pretty much unobtainable these days. The transmission should shift easily and smoothly. Do not buy a bike that jumps out of gear or shows false neutrals.
Aside from this, the usual precautions apply. Check for oil leaks, crash damage, missing parts etc.
Oh one other thing. A mk1 NSR has a slightly different instrument cluster than later models. If a later cluster is fitted to a mk1 the low oil warning light will appear to function (light for 10 seconds when the ignition is switched on) but it will not light when the tank is empty. Always check the function of this system or you will seize the engine!