Chris,
ECVs can be rented from the International Gateway at EPCOT. However, the ECVs are not always available throughout the day & evening. when a guest returnes an
ECV, Disney recharges it, so it is not always available right away.
I would definitely rent an ECV from an off site supplier. I'm sure Cheshire Figment will be along & share his info about renting off site. Or read other threads here about ECVs, & you'll come across his posts.
I started using an ECV at WDW by renting one at the parks. I probably rented from Disney for about four trips. However, renting from Disney presented a few "issues:"
*no guarantee the park will have an ECV regardless of time of day (I was at EPCOT Mother's Day 2002 a half hour after the park opened & no ECVs available at either entrance);
*if you want to park hop there is no guarantee of an ECV at the second park;
*four wheels on a Disney ECV vs. three on an off site ECV (at least the ones I have rented from Walker); three wheels gave me a better turning radius;
*battery holds up much better on the rented ECV vs. the Disney one; I can tell you where in each of the four parks my Disney rented ECV "died;"
*if you rent an ECV you will have it every where you want it - the parks, Downtown Disney, other resorts, etc.
The last several trips I have rented from Walker Mobility & had a very good experience. The ECV has been delivered in a timely manner; the battery has lasted all day & night; the cost is similar to Disney, & I have the ECV more than just in the parks; etc.
As for using an ECV - go for it! You have a need & the ECV will make your trip more enjoyable.
On my last trip we stayed at OKW & I used the busses to the parks. I was hesitant, but the bus drivers were so friendly & very helpful.