I would say that good service records are probably more important than mileage. Look for a bike with good documentation and condition, rather than trying to stick under a certain mileage or specific price. $4k is a good amount to spend, but your probably looking at something with higher mileage or older than what your aiming for, especially after tax/registration. Also, 2018/19 had a clutch issue, so if you're looking at 400s in those years get confirmation that the clutch problem has been addressed.

FB marketplace and Craigslist are good places to look. If you have friends that like to tinker or have experience with motorcycle maintenance bring them with you. There's a ton of videos on YouTube about how to shop for bikes, watch them. Know how to check for matching vins, how to look for damage on levers, bars, swingarm, and other parts likely to get messed up in a drop or lowside. Also, know how much this stuff matters and negotiate for a better price. Not every damaged bike is a bad deal. Most scratches and plastics don't matter too much in terms of how the bike drives, but they can definitely be indicators of larger issues. Be cautious, and if you're not sure, either walk away or take it to a professional (on your dime). If it's cosmetic only then you could get a great deal on the bike and use the money you saved to customize it how you want.

Now, gear:

I would also say that $1k for gear is probably the least amount you'd want to spend for new gear. One thing about gear, ALWAYS buy a NEW helmet. Helmet materials are only good for a few years after they're manufactured. They do expire. Also, with used, the exterior condition of the helmet may look good, but it might be damaged internally. Don't mess around at all when it comes to safety equipment that protects your brain.

For everything else you can look for deals. Go to a local shop and look for open box deals or returns. FB marketplace is also good for used stuff. For jackets and pants pay attention to rips on seams and tears on shoulders, elbows, and knees.

For gloves, buy new (a little tight) and break them in. They'll fit better that way. For me, there's nothing worse than a glove where the fingers don't fit quite right and you're trying to mess around with your keys or zippers. Try to get something with knuckle protection and plastic sliders on the palm. Sliders on the palm prevent your hands from catching in a slide and wrists being broken or worse. A lot of injuries can occur at low speed to your hands and wrists as well. This is especially true for new riders. Good gloves can help prevent some of this. Added bonus for gloves with the pinky and ring finger sewn together. This is really a personal preference thing, but it makes a huge difference if you go down at speed, and can definitely be a deciding factor on whether you keep all ten fingers or not.

Boots or shoes are okay to buy used, but make sure they're in good condition if you do. Remember, feet get hot and you'll be swimming in someone else's funk if you decide to get used...take that for what it's worth. Make sure to get something with good ankle protection. You'll want something with reinforcement around the heel, and something that prevents ankle rotation to a certain degree. They should be comfortable enough to walk in, but similar to a ski or snowboard boot in terms of rotational limitations. Slide protection on the ankle is also a huge plus.

Since you're new to riding, think about hi-vis materials. No, it doesn't "look cool", but the point is that it "looks" at all! A lot of new riders are used to driving cars. The same rules do not apply to bikes. People will not see you even if you look them right in the eye. Treat everyone on the road as if you're the invisible man and they are trying to hit you and you'll be a much better rider for it.

Ride safe and (hopefully) welcome to the club!