The cost for the Disney
ECVs are $30.00 per day plus a $10.00 refundable key deposit. If you park hop, having your receipt showing the daily rental will allow you to only pay the key deposit at another park
IF one is available.
Walker's rate is also $30.00 per day, but it caps at $199.50 for a full week. Care and Colonial are also in the same range; Randy's is less expensive.
All except Randy's will drop off the
ECV prior to your arrival (so it will be at Bell Services) and pick up after you depart. Randy's requires you be present at both drop-off and pick-up. I have used Walker several times; their policy appears to be: If you are arriving early in the morning they will drop it off the afternoon before and not charge. Similarly, if you are arriving in the evening they will drop it off in the afternoon and not charge. Usually if you are leaving in the afternoon and are done with it by about 1:00 they will only charge a half day for the last day.
Note the off-site rentals are three wheeler's, which are much easier to maneuver than Disney's four wheel ECVs. If you have not used one before, you might want to try one out in a large grocery store, realizing that the rentals are much easier to use, especially since they don't have a grocery basket.
You should not have any problem with Disney Transportation. From BC it is easiest to drive the ECV to Epcot, take the boat to MGM, and buses anywhere else. If you have a car, the ECVs will break down into five pieces, the heaviest weighing about 39 pounds. Having a car is convenient, but to use any of the medical parking you will have to have an HP hang tag. I find it easiest to get anywhere OTHER THAN the Magic Kingdom by car.
I hope this information helps.