Is Hogwart's Express a must do?
It's a must-do, if you're a Potter fan. If you don't care about HP, it's not a must-do, but it is well done and enjoyable. You're also there for 5 days, so it would be a tease to have two rides (it's different in each direction) that you can't access. UO has fewer rides than WDW, so don't eliminate two of them before you get to the park.
If you're used to WDW, you are probably overestimating the size and distances at UO. The parks are literally right next to each other. Sometimes there's a walking cut through open that allows you to park hop in less than 5 minutes. UO is very compact, and the premier hotels are very close. You will easily be able to go between the parks and your resort.
WDW rewards planners. UO is a more spontaneous experience. You don't need to make any ride reservations. If you're used to the old FP+ or the new Genie and lightening lane, UO might seem a bit strange. But trust us when we tell you that you'll want to be able to just go wherever you want whenever you want. Park hopping gives you the chance to ride your favorites everyday.
For a 4-5 day trip, the cost of a park-to-park ticket is not much more than a one-park-per-day tickets. And as others have mentioned, an AP can be about the same price. Plus, if you can snag a pass holder rate at the hotel, purchasing APs might be less expensive overall.