I hear you about the temptation to get an all-in-one setup. That said, the money you would spend on a decent boombox ($100+), you would be better off with your original plan.
If you went dirt-cheap with your buddy's receiver and some speakers from Parts Express, all on the same table next to each other, it would still sound 10X better then a high-end boombox. The advantage is that if you could find a way to get the speakers a little farther apart (hanging from a screw), you would get even better sound.
Dayton Audio B652 6-1/2" 2-Way Bookshelf Speaker Pair 300-652
You could also consider these speakers as they have more oomph and are super durable and easy to mount.
Dayton IO650B 6-1/2" Indoor/Outdoor Speaker Pair Black 310-056
Besides, weight lifting is much easier with loud, aggressive music such as Rage Against The Machine
good luck,
Jotham