Scooter motors or wheelchair motors will be fine. With wheelchairs you get motor/gearbox/wheel in one package. Lots of wheelchair units are scrapped when they have lots of life left in them. Go to your local mobility store and see whats available or check the ads in Friday Ad. You will need to remove or defeat the brake but thats not hard.
Wheelchair units will give you a slow and underpowered robot but it will be an easy build and easy to drive. You can run 36 volts to the motors to speed it up. Make it very solid with armour over the wheels because although you plan a middleweight, you will find yourself fighting heavies. I have had over 50 fights with my middleweight and only 4 of those were against other middles.
Good luck with your build.