The first time we used a wheelchair at WDW was unexpected (DD13 broke her ankle) but we had already booked HDDR category 3. They had tables at the back on the ground floor for people who would otherwise have to be upstairs but couldn't get up (she chose to hobble up the stairs because she wanted to be upstairs like we booked but we were given the option of a table at the back if she couldn't do the stairs). Call to book instead of booking online. I actually don't think that's legal to require you to book the most expensive category just because you're in a wheelchair so you should be able to book category 2 or 3 if they're available. Heck, half of category 2 is on the main floor anyway. Do tell them that you need an accessible table but there's no reason for you to be forced to book a more expensive category when there are accessible tables available in other categories.