couple of things...
as a beginner, you'll find more time in driving 'correctly' and basic kart setup than the difference between a new chassis and 2nd hand one, so 2nd hand is good, just take the advice above and look at the overall condition. Another common practice here (Oz) is to buy a new engine and 2nd hand chassis to ensure your engine is both legal and not on it's last piston size!.
Also, remember to factor into your budget your safety gear (helmet, suit, gloves, shoes) and a couple of specialised tools that you need like T-Handles/whatever to get rear wheels off, alignment bars, spare stub axles
Having to add a few kgs of lead to your kart is ideal, cause if your too heavy you're disadvantaged. Ideally, hit the scales just over the class limit (after a race so make sure you allow for used fuel).