definitely avoid any suspension components at that price.
when shopping for a used bike, the most important thing to check is the condition of the wheels. spokes should be stainless steel and not plated plain steel. rims should be straight and not have any dents or 'taco' flanges, their shouldn't be more than a millimeter or so of side to side when you spin the wheel (watch the rim to brake pads distance for an indicator of this). there should be NO up/down motion. smooth side to side motion of a few mm can be adjusted out by someone who knows how to tune a wheel as long as the spokes aren't corroded. wheel bearings should be smooth, with no play. as long as they aren't 'bumpy' from pitting, they can usually be regreased and readjusted very easily to fix minor problems. the 'bottom bracket' bearing between the crank arms should also be smooth, with no play (wiggle the two crank arms side to side, if you feel any clink/clunk motion, the BB will need some attending to, not at all hard if you have a couple speciality tools that will set you back maybe $40 total and tools last for a lifetime)
turn the cranks, and watch the front sprockets ('chainrings') relative to the front derailleur, there shoudl be NO side to side motion of any of the chain rings relative to the derailleur. ride the bike, and shift through all the gears (don't use both smallest cogs, and don't use both biggest, those two positions are called 'crossover'), and make sure the chain doesn't jump, especially in the tallest gear (big ring in front, smallest cog in back). a stretched and/or rusty chain can be replaced for $15 or so, but often a little cleaning and oil will do wonders.
quite common on a old used bike for the brake pads to be worn out, hard as rocks, and/or very badly adjusted, thats cheap and easy to fix, ditto corroded cables that don't brake and/or shift well, also really easy to fix.
worn out tires are easy to replace, figure $20 each for really cheap tires, $40 for much better ones. add $5-10 per wheel for new inner tubes. If its a mountain bike that you plan on using as a commutter, you'll want to get non-knobby tires in a relatively thinner size, like 26x1.75 slicks instead of the more typical 26x2.125 knobby
if the rear derailleur cage isn't parallel to the cogs, the hangar can usually be bent to fix that, and the gears will work much better.
MOST important thing that can't be fixed, the frame should be the right size for you. I"m 6' tall, and ride an "L" frame in most all bike styles and types.
on Lon'Gisland, there's likely a 'bike co-op' kind of place where for a very nominal fee, you can join and use their workshop and tools, and their experience.