I would be looking into something like sliding door opener..
200W is more than enough, provided gearbox (with worm gear) is adequate (heavy duty).
http://www.ebay.com.au/itm/Nice-Robuskit-Slide-Gate-Operator-Motor-Kit-110V-24V-/120701548700?pt=LH_DefaultDomain_0& hash=item1c1a5f789c
We have rep here in Oz (for Nice door opener)
EDIT: here it is:
http://www.niceforyou.com/en/2-sliding-gates.html
Or, second-hand windshield wiper motor from big truck.. for example something like this:
http://www.ebay.com.au/itm/12-volt-m...item2a28d6eb65
Not much more than $50 ~ $100, me thinks.