Breastfeeding on formula?

my son will be born jan 28th n i was thinkin of switchin him bak n forth between my milk and formula. Is this ok for him? im only breastfeedin for about 2 months bcuz it saves money and i heard its a good way for bonding time. And if so wat ump shuld i get. I was thinking of geting the medela manual pump. Well let me kno. Thanx!