just make sure the arm warmers don't pinch you in the biceps. I'm not a really big person but boy howdy do I have biceps. The guy at my LBS took a look at my arms and said - large! and he was right. I put them on and flexed my biceps to make sure they wouldn't pinch. I have some PI fleece lined, the Primal Wear tattoo warmers and a pair of very special one-of-a-kind Amici Veloci arm warmers! I don't use a helmet cover, I wear a polypro balaclava becuase my issue is with my ears getting cold and then causing me no end of problems. The balaclava works very well keeping my head warm also and they aren't too expensive - it might be an alternative for your budget.
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, champagne in one hand, strawberries in the other, body thoroughly used up, totally worn out and screaming: "Yeah Baby! What a Ride!"