ok, in which case you will still need a bb and front mech, at the least. if the front mech is band on (and not a brazed on fitting), you need to make sure you get the right one for the thickness of the frame tubing (alu is commonly 27.2mm, but check to be sure).
changing a bottom bracket is easy - if you have the tools; crank puller and bottom bracket tool with large adjustable spanner and allen key.
if this is a one-off job, i'd recommend a reputable local bike shop to do it, otherwise get the tools and do it yourself. if you're a member of a cycling club, you might get lucky and be able to borrow the tools…