The motion can be defeated though right?
Yes. I would only do this if you have got the numbers having talked to neighbours. It is a way to force the hand of the EC.
I have been discouraged in the past to add a motion because it would have to go through the OC whereas I was told that if we informally discuss the plans, then the EC could allocate funds within permissible limits for the gardens.
Yes. If the EC could be persuaded to use some of an existing budgets for grounds maintenance, that would be preferable. It depends whether there really is sufficient money in the budget or not. The motion would be a way to ensure there was enough.
Is the OC obliged to keep the gardens in a state they were when I bought 15 years ago?
There is a general obligation to repair and maintain any aspect of the common property and that includes the grounds. That might not mean exactly as it was 15 years ago but the gardens should not be simply neglected. An advantage of a motion could be to set out what the OC wants its gardens to look like. Some people might be happy with gardens that look a little wild while others might want lawn you could play bowls on. It depends what people want and that could change over time.