If you check out the disABILITIES FAQs thread, there are a lot of things there that will help you to maintain your sanity.
Posts 18-21 are about access. They list all of the attractions with accessible ride cars and also what kind of transfer would be required if a transfer is required.
WDW is very accessible. All of the buses between the parks and resorts, all of the monorails and most of the buses are accessible. The Disney's Magical Express has some buses that are accessible with lifts; since not all are, guests needing one might have to wait.
And for the question of weight - a power wheelchair is MUCH, MUCH heavier than the average
ECV. My daughter's power chair weighs more than 250 pounds with or her in it. Despite the weight, most power chairs will fit into the same space as a manual chair and since the majority of people using them can't transfer or transfer with difficulty, they are equipped with tiedown loops for transport.
Many ECVs are larger in length or width than a powe chair and most are nowhere near as maneuverable. So, they either won't fit or cent be driven into the wheelchair spot. And, for attractions like TSMM, they lack tiedown spots, so can't be fastened in.