yes, manamune+berserkers allows you to charge it up very fast. i get bout 2.5k mana by 30-40ish minutes (spam whenever skills are off CD, auto attacks). and it does give you AP from the AD you get anyways, and also makes farming a breeze. if you decide to go ap, grab a rylai's, it gives you permaslow, then rabaddon's. typical ap build for me would be nashors, berserkers, manamune, rylai's, rabaddon and lichbane. if i go AD, i switch rylai's and rabaddon's for 2 bloodthirsters, and lichbane for triforce.
Note: in either build, your heal is useless for its heal, USE IT AS A MS BUFF!!!