I'd forget a cordless angle grinder, they don't last long on batteries and most brands really struggle to cut anything decent.
i have had a number of battery grinders. dewalt,milwaukee,makita,bosch. The best cordless grinder imo is the dewalt with nicads. With l-ion batterys they tend to cut power to avoid overheating the battery's so tend to stall on hard materials. L-ion batters are good but they have there problems and if money is tight i would suggest sticking with older tech as the batterys last alot longer and are very cheap to buy.