You might get it for lesser, But if it goes kaput after some years, The chances to get it fixed will be zero when compared with the MGA Unit. My car's winder Unit went bust on the driver side some time ago and as it was an MGA unit, I got the part replaced and it works fine. If it were some other brand with little or no service support, I would have lost all the 5500 by now.
If service and spares are available freely for the cheaper one, Go for it. If not stick to the genuine product.
Yes fiiting power windows is possible for almost any car. Get in touch with accessory shop, he will let you know about the exact costs involved in fixing the unit.