I've stayed at the Radison Resort Parkway without a car, and would consider the shuttle service acceptable, but not great. During the day the service is relatively infrequent. So planning to take rest breaks during the afternoon was difficult.
If you are going to stay on I-drive, search for a hotel that has a dedicated shuttle so that you're not spending 30 minutes stopping at other properties on your way to WDW. It's your best bet for arriving at the parks before they open.
You might want to consider splitting your stay between a WDW area hotel, and a Universal area hotel. Staying on-site at Universal gives you "Front of the Line" access, and so you can get a lot done in two days if you stay on site at Universal.
Check out the transportation boards, there's good info to be found.
Good luck!