Congratulations on your purchase. The Rebel is a very dependable ride and easy to maintain.
If you can turn a wrench, you can save the cost of someone else changing the oil for you. It's easy to do, and there are videos explaining just how to do it. BTW, changing oil in this air cooled engine needs to be done frequently. Most members of this forum change it every 1,000-1,500 miles. With frequent oil changes, the engine will last over 100,000 miles. There is no oil filter on the Rebel 250. If someone tries to tell you they changed it, they aren't being honest.
Unless the carb and tank were drained of fuel, both will be seriously gunked up and will need thorough cleaning. If you look in the Wrenching forum, there is lots of info on cleaning tanks and carbs. There should be no reason to rejet the carb if it is cleaned well.
If you can slightly dent the tres with your thumbnail, they are probably ok. If hard, replace them. If the tires are original, they probably need to be changed. Look here for how to determine the age of a tire: https://www.tirerack.com/tires/tiret....jsp?techid=11
Occasionally, the valves on engines that sat as long as yours will stick, requiring major repair if they aren't freed up before riding. Remove the valve cover, lubricate the valve stems, and turn the engine by hand to be sure everything is moving freely before cranking the engine.
Unless mice got into the air filter, it should be ok. Putting a K&N type air filter on it will do harm and no good. K&Ns allow more dirt to get into the engine, and won't produce any more power, other than that reported by those using a "butt dyno". If anyone would dyno their bike both before and after the air filter swap, I might change my mind. Also, getting rid of the stock air box eliminates the crankcase breather feature. Some put a small pod filter on the crankcase breather hose, but it will eventually plug up with oily condensate on the inside where it can't be seen, raising pressure inside the engine to the point that it blows out the left side oil seal, allowing the oil to escape.
You can remove one or two of the baffles in the stock exhaust, but it's not going to rival the sound of a bigger bike. Ditto with aftermarket mufflers. Many who have done exhaust mods report that it sounds like an angry lawnmower. There's no way to get a big V-twin rumble from a Rebel with its small, parallel twin engine.
I'm in the camp that thinks louder pipes don't save lives, they're just annoying. If you haven't taken a basic rider course, you should. Studies show that trained riders have fewer accidents compared to self taught and friend/relative taught. It's far better to learn how to ride defensively than trust in a myth to help keep you safe. One mod I do recommend is to get a louder horn so you can be heard. The stock horn is about as loud as the Roadrunner in the cartoon.
I was writing when SS posted his response. You can see a lot of similarities in these posts