Smart up to how much km? What do you pay attention to when buying?

Good day, I would like to buy my first car (automatic) with a small budget soon. I would like to have a smart for the city, but I don’t know exactly what I have to pay attention to, except that it should be taken care of by cheques? I think my budget is at a maximum of €3,000, it will be the 450. I have looked at eBay classified ads, mobile de, and autoscooter, but I’m always skeptical because I’m afraid to be pulled over the table. Unfortunately, I don’t know anyone who knows about cars well. From what kilometer is no smart any more? And what price is still justified? I see many smarts 450 with 90.000+ km for 2,500€, is worth it? With private providers there is never enough information, and also many online dealers without check booklets indication. If you think that it’s not worth it for the number / price, I’m also grateful about cheap suggestions about other car models.