you can probably gt a lab coat at some costume shop for the 'evil scientist' look, just make sure there's no fake blood in the coloring of it,
for the gloves i'd just take some normal latex gloves, paint 'em red... I dunno if it'll work but....
for the medi gun and the backpack, i honestly have no clue.. i'd just experiment,
http://www.cosplay.com/showthread.php?t=128928&page=2the third comment on this page goes into a little more detail of what you really need for the medic costume, especially the medic gun.
good luck to ya and I hope I helped a lil' bit.