I like the Bachmann Davenport suggestion but it is a small engine and short on space for your electronics. I think the better suggestion is the MACK. You can buy one for as little as $50 and there is lots of room under the frame for adding weights and you could rearrange the cab and motor covers to make something really unique.
A 8% grade is pretty tall. Is it all straight track? If so maybe you could build some type of cable system to haul the cars up?